Other M's definitely not the best Metroid game (a title that I have to give to Super Metroid or Metroid Prime), but it's a pretty solid entry into the series. The game has fully 3D gameplay, but it's in a tight environment that, often times, is very reminiscent of a 2D environment. It's a brilliant balance between 2D and 3D gameplay, and it truly does feel like taking the gameplay of the 2D Metroid titles and putting it into 3D. It also combines some key elements from the Prime games, which are the 3D Morph Ball sequences, as well as first-person shooting. There's some annoyances with switching between the standard movement and aiming modes, as well as with being unable to move while aiming, but they're not something that you can't easily look past if you're enjoying the game.
From the game design perspective, this is one of the most linear Metroid games. The game's presented in a similar way to Fusion in that you're constantly ordered and guided along a set path as part of a plot, but it's even more restricting than Fusion was. Still, as you near the end of the game, pretty much the whole map is available to you. The similarities to Fusion continue in the plot; it's not the most original game out there, and if you've played Fusion it will feel very familiar. There's some cool fanservice for Metroid fans (such as my favorite boss from Fusion returning), but a lot of them come out of nowhere and don't make a lot of sense within the continuity of the series. Cool, but not intellectual.
As for the story... it's essentially good at base. There's a strange bio-engineering plot like there is in any Metroid game, with secrets to reveal like in the Prime series. It borrows Samus' history and many of her traits from the canonical Metroid: Zero Mission manga (the much complained about scene where Samus freezes up is basically a direct tie-in with an event in the manga; if you want to read the manga, you can check out both
Vol 1 and
Vol 2, the complete manga, on Metroid Database). The voice-acting is essentially good, though it's not the actual VOICE I would have chosen for Samus. The writing, however, is pretty bad. It's a cheesy, hokey story, not because the story itself or the events that happen within it are bad, but because it's mishandled by the writing team. It's still enjoyable, though.
The only serious problems with the game aside from that are the spotting sections -- in which you must use first-person view to spot some object in the environment -- which are frustrating and just not fun, as well as the final boss fight where your equipment is suddenly allowed to be used without any warning, and you have seconds to figure that out or die.
All in all Other M is a very entertaining game, and it's basically got a lot of its ideas down perfectly. There are most certainly flaws, but not enough to drag the game down and it's definitely worth a shot. I recommend it, especially if you found it at a good price.