Lets get the first immediate cons out of the way. It basically costs as much as a Topre keyboard (near $265 USD or so), and the wired version (there's a wireless version as well) has a anxiety inducing thin wire. So for cheap and portable use, this ain't it boss. For everyone else, or everyone else still curious, here we go.
Note that Contour, the company that makes these does offer a 30 day trial for those interested but not 100% sure if they want to spend the money. Since reselling these things is a bit of a risk with how niche they all are unlike Topre keyboards which have a relatively high amount of people willing to purchase a used board.
As for how it could be used, this is one of many hand positions that can be used with this particular model of RollerMouse:
With all that out of the way, continuing on.
Build quality: It is damn near entirely aluminum and it is very solid. So besides the extremely thin wire, everything else is fairly solid or (mostly) acceptable at worst. The worst in my opinion, being the finish of the mouse buttons.
This photo album: https://imgur.com/a/H0E3atk
has more pictures of different ways it can be used. Along with visible damage to the mouse buttons' finishes. (I played Terraria a bit too much lets just put it that way)
Note that the hand movements are exaggerated for visibility reasons. Realistically the movement in the photos would be much more subtle and almost unnoticeable with the camera I was using to take those photos, hence why they're so hyper exaggerated.
It has several DPI settings, those being:
600, 800, 1000, 1200, 1400, 1600, 1800, 2000, 2200, and 2400 DPI respectively.
Mines on 1200 DPI. This can be changed without the use of external software.
The external software that can be downloaded via Contour's Website (Link: https://www.contourdesign.com/product/rollermouse-red/ it is on the bottom of the page) has several useful features. Besides the typical "cursor speed", "cursor acceleration; on/off", etc. It has things like re-programmable/rebindable mouse buttons. Unlike say most keyboard external software, this one is super easy to use that even my dense ass can manage LOL.
It even allows for application specific profiles. So if I start up Terraria, it will automatically use the profile I set up for that particular .exe file.
This is what the software looks like, along with my personal settings with the top screenshot being the mouse speed settings, and the bottom picture being my mouse button bindings.
If you're wondering why the rollerbar itself is bindable, that is because there's a optional (as in it can be turned on or off) "tap to click" feature. The sensitivity, and the loudness of the click can be modified without the software. I usually have the click turned off along with the tap to click feature all together but I sometimes use it.
Oh, and the weird black barrel looking thing in the center of the mouse button cluster is the mouse wheel. Despite how it looks, it is about as smooth as a CST trackball's scroll wheel. It can go deceptively far as well. It can also be clicked, and that means it can be reprogammable just like the other mouse buttons.
The thought process being my bindings is that, if I have tap to click enabled, no matter if I use my left or right hand, the "left" and "right" clicks would be symmetrical so I can swap hands without too much issue.
It can be cleaned by pressing those two black plastic pieces on both ends of the roller bar inward.
Here's what that looks like: https://imgur.com/a/50qX9BA
The actual button cluster cover can be pryed off with a flathead screwdriver. I do not have photos of this but Contour does on their support page for the RollerMouse. Its under something along the lines of "2. My buttons are sticking" or something like that.
As for the actual feel, holy fucking shit it is smooth. Smoother than my CST with a high quality pool ball. It is much lighter to use as well whereas the CST is a bit on the heavier side to roll that pool ball around. It takes about as long as it takes for a CST to get "dirty", so about 4 months of casual use before the grim starts being noticeable. Around that time is when I clean mine and I always have a fun time using a newly cleaned RollerMouse with how smooth it is. No break in period required!
One really peculiar trait is the lack of cursor drift. Because my main game of choice is osu!, I immediately used the RollerMouse RED in that game the moment mine arrived, some time in the year 2018 give or take. Ever since that day, I've always used it for osu! because of the lack of cursor drift. Not having to place a tablet at either end of my keyboard also streamlined what I needed placed on my desk space at all times. So for games that either have a locked camera most of the time (like Guild Wars 2), or outright doesn't move like osu!, this is surprisingly stellar for those kinds of usage!
Because of the way it is positioned relative to the keyboard, this also has the implication that keyboard horizontally speaking, doesn't matter with this "mouse". So use as huge of a keyboard as you want! (Horizontally speaking)
Also, again, due to the position of the RollerMouse RED, you can actually somewhat use your thumbs to use it even if your other fingers are resting on the homerow of the keyboard. So you can use it almost like a trackpoint in a way. Do to this, and other traits like the tap to click, I like to look at this mouse as a sort of jack of all trades master of (almost) none type of deal. Its for people who need a specific niche of features from other families of mouse types that cannot be found easily.
Do I recommend it? To most people, no. This is too niche for the general public imo. I used trackpoints, trackballs, touchpads, tablets, mice, etc. and I wanted something that had a bit of each of those. This is the closest I can get to that and have it be done somewhat fluidly. Not to mention that my experiences with all those devices are super rare in that combination so I am fully aware that I am in the minority and I will not pretend that device will work with a general group of people's lifestyles. So for most people, a mouse or a trackball will do. For people with those super niche things they want, this might be it.
There's similar devices like this one such as the "MouseTrapper" line of mice. Those ones are basically mechanical trackpads (the newer ones at least). I have no experience with modern MouseTrappers so I cannot confirm whether or not they're good, but I'll mention them in case you want to do some research of your own.
PS: Note that the armrest comes in three types that can be purchased from Contour or their Amazon storefront.
The default small wrist rest. The extra large wrist rest. And finally the arm wrist rest which is meant to be used with standing desks but I like it so much that I use it regardless of which desk I'm using. Do note that unlike the other two wrist rest options, the armrest doesn't "click" into place like the other two do. The armrest, instead, clamps on to the desk and isn't permanent so it can be removed and reattached as needed.
PSS: It is compatible with Macs, Windows, and allegedly Linux PCs. I do not have experience with Linux however so the only two that I can confirm works with this "mouse" is Windows and Mac respectively.
PSSS: Dammit, I keep forgetting to add these things LOL. But the RollerMouse RED comes with two keyboard risers. They can be tilted either towards you or away from you. So "negative tilt". Or you can have both tilts enabled so it just outright rises the keyboard altogether while still being at "neutral" tilt. I have mine on negative tilt so my hands can "hug" my Unicomp EnduraPro a little bit better. The manual says that the keyboard's spacebar should be higher than the rollerbar. That is the only major thing the manual recommends you do with regards to positioning the RollerMouse RED relative to the keyboard.