I had avoided this game for a long time. Recently, however, I've been scooping up just about all the old Sonic related games and when nearing the end of the barrel finally decided my curiosity for this game was worth exploring. It's worth pointing out at this stage that just about the only the games in the series I haven't liked enough to think I'd like to play them again at some point were Sonic 3D and this game's follow up mainly known to fans as Sonic 2006. So, yes, I did play and enjoy even the divisive Sonic Heroes and Sonic Unleashed. Readers should now be well versed enough to qualify my opinion against their own, hope that background helps.
Much comment has been made about the guns and vehicles in this game, and I'll readily admit that these were the primary reasons for me putting off looking into this game. Having now played it though, vehicles hardly feature in the game at all ecept as an option you'll almost always run past without even stopping to consider and although the guns are a prominent option they feel very natural in the game. In fact, gameplay feels pretty consistent with Sonic Adventure 2 in general.
Marketing has boasted something like 326 possible stories in for each playthrough of the game, between which there are 10 possible endings. However it should be noted that many cutscenes are shared across multiple 'unique' stories, making it feel as though really there are only 5 stories as would initially appear to be the case when looking at the story progression chart, in game.
The most enjoyable way to play the game, I felt, was to stick to the mainly neutral path where Shadow is required mainly to reach the goal on each level. To reach some levels, however, Shadow has to forge an allegiance to either the dark or hero characters which requires fulfilling missions that only occasionally vary away from beat a certain number of enemies or find 5 locations. The problem however comes when some enemies are tucked away in quite large levels. When this is the case gameplay still feels reminiscent of Sonic Adventure titles, but more in line with the infamously divisive emerald hunting levels. (If you did play Sonic Heroes there were also a few levels there that required Team Chaotix to hunt down a certain number of enemies or in one case hermit crabs, you'll be familiar with the potential frustration. If you didn't play Sonic Heroes I would add that was probably the weakest element of that game and shouldn't put you off taking a look at the rest of it.) A mostly neutral playthrough of Shadow the Hedgehog probably won't take players much over an hour (comparable to the classic Mega Drive games that fans constantly request similarities to) and players could well enjoy a few trips through this with differing paths, but it is going to take a more dedicated player to spend maybe 45 minutes per level on some of the more committed dark or hero path levels, hunting down the last two enemies in order to progress without moving back toward the neutral alignment.
As a side note, I'm running my copy on the 360's emulation software and thought there might be someone who finds some feedback on this helpful. For my first playthrough of the story the emulation on the 360 was seemingly perfect, so I was initially quite pleased. However, after further play it seemed that the emulation is rather prone to crashing at the beginning of cutscenes or in the first 10 seconds of some levels. This seems rather more common (almost guaranteed, in fact) when loading an unfinished campaign leading to a need to restart the campaign if not completed in one sitting or if you've had the bad luck to have a purely random crash. I don't factor this into my rating, above, as obviously the title was intended to run on the original Xbox, but if you were looking to run it on a 360 be aware that it can be a fair bit more frustrating than you may have expected. On the upside, I've not noticed any disrupted speech, dropping framerates or slowing gameplay that other emulated titles have given. It's also worth Xbox owners being aware if you've got an alternative console that the other two console's versions of the game at various points allow a 2nd controller to play as your sidekick in the campaign - whereas that feature is inexplicably absent on this version.
In summary this is a very fun game (particularly at the price it's currently listed at), although wasn't a technical marvel even at release date. Despite multiple story paths, however, if you're not a hardcore fan of the genre (and particularly if you're emulating on a 360) might only last you around 10 hours.