The Super Mario Bros. Game & Watch feels a bit small in the hand but I don't find it hard to hold or unreasonably uncomfortable. The screen is plenty bright and the one speaker seems loud enough for me so much so that I've turned the screen bright and speakers down a bit from the factory default settings. The whole user experience of the Game & Watch is well considered and the Game / Time / Pause Set buttons work quite well in navigating it.
The Game & Watch includes Super Mario Bros., Super Mario Bros. The Lost Levels and a digital recreation of the original Mario Game & Watch game. As an acolyte of the first Super Mario Bros. this was the biggest draw of buying the Game & Watch beyond having a fun collectors piece. In a world where one owns a Nintendo Classic, Switch -w Nintendo Online and the original Super Mario Bros. NES cart is there a reason why one would play the game on the Game & Watch? The answer is yes as just yesterday I completed a full play though of each level of Super Mario Bros. on the Game & Watch and I plan on playing it a lot more as a result. My only minor complaint about the Game & Watch controls are that the A & B buttons are smaller than the original NES controller versions. I'm not sure this amounts to really anything significant but those original controls are forever burned in my brain so I notice it whenever I play. Super Mario Bros. itself is emulated on the Game & Watch and it's done so well that I can't really tell a difference in that I don't notice any emulation lag.
The Game & Watch has a lot of fun secrets to it and I've included a link below for you to check them all out. Now that I've beaten Super Mario Bros. I've unlocked the ability to switch into hardmode when I want and start at any level I choose. This is a fun unlockable feature that was available in the original after completing the game but due to no save states would be lost when you reset your Nintendo.
If you are looking for a fun Mario collectable you'd do well to check out the Game & Watch. It's not particularly hard to find right now and Nintendo will stop manufacturing it on March 31st of 2021 so if you are interested I'd get one while you can. While the Mario Game & Watch came to usher in Mario's 35th anniversary this year will mark the 35th anniversary of the Legend of Zelda. Given how fun this handheld is, I hope Nintendo does something similar for Link!
If you want the IMMERSIVE Game & Watch experience you should consider getting the stand for it. I did and it's a great thing to have hold the watch up on your desk.