Tried this character out for the first time today; after completing an all-emeralds run, I feel confident in saying that this is one of the most fun custom characters I've tried.
The sprites and animation are excellent and fit right in with the vanilla art style. I played with music turned off today, so I really appreciated the skating sounds. No complaints in terms of visuals and sound design.
Having played through the standard vanilla 2.2 campaign, I can safely say that Shadow's moveset is both fun and well-balanced for the game. There are some pretty wild shortcuts you can take, but they require some mastery of the chaos snap, and in standard gameplay, the chaos snap feels neither overpowered nor underpowered.
I'd say the chaos snap feels like a more controllable (and cooler-looking) thok, particularly since you can snap backwards or upwards to recover if you overshot your destination. (And I'm saying this as someone who's actually become decently skilled with Sonic's thok.)
The ability to aim your snap upwards when not holding a direction is a good way to squeeze multiple actions out of the limited button set. It reminds me of how the spindash and peelout work in the Genesis games.
There is one issue, though, and it's the reason why I can't give this a perfect rating. The chaos snap would often end up bonking me into a wall/ledge with an inaccessible monitor nearby, due to its homing ability. I don't know how feasible it would be to implement (I've noticed that Modern Sonic has similar issues), but stopping the snap from homing into objects you can't actually reach would help a lot here.
I commend the choice to limit the character to vanilla-style controls (only jump and spin buttons being used), and in general, I think it succeeds at that with flying colors. The unwanted homing bonks are just a little too frequent for my taste.
(Yes, I am aware of Shadow Plus, but I am rating this add-on as its own thing.)
Overall, this is an excellent mod, and if I were to compile a list of must-install add-ons, you can bet that Shadow would be included.
POSSIBLE SPOILER FOLLOWS:
I'm really glad I didn't read the reviews beforehand; the easter egg caught me by surprise, and I must admit: I laughed. I'd have to disable that before letting kids play with the character, though; I wouldn't want them to start quoting it!