It's still early days for the DS, this is a good sign of things to come. It's nowhere near perfect, but worth the money.
I was probably feeling a bit nostalgic (and i'm not even 30 yet) but when i first booted this one up, it got the hairs on my neck standing, the music, the graphics, the level design. It's easy to see that this has been polished, like any A-list Nintendo game. When you first start playing, the control system (using the stylus like a mouselook) feels totally weird, but i'm now at the endgame boss, and it feels second nature, but still a little niggly. Very often I end up doing stuff i didn't mean to, like turn on the scan visor or jump when i meant to walk. This can happen at critical moments like when you need to traverse some moving platforms, and it really pissed me off when that happened. Other things i noticed were the very grainy textures. I know this is an early title for the DS, but i kept wishing it looked a bit more modern in terms of graphics. Can't the DS even do bilinear filtering?
The best thing about this game is probably the level design. It is typical Metroid. Puzzles, sequence-breaking, traveling back and forth, unlocking new areas, picking up new weapons along the way. It's surprisingly fun to play a 3D Metroid on a handheld. Don't expect too much though, this game does have many problems other than those i've listed, such as the bosses (all the bosses except the last are just variants of two actual bosses, looks like Nintendo ran out of boss ideas here), the crappy music, the crappy 16-bit style sound-FX, the lame enemies, the sometimes confusing layouts of the levels, the uninspired choice of weapons (they almost all feel the same), the way Nintendo didn't make good use of the ABXY buttons (they are all mapped to jump, why Nintendo why??)
Good points: the 3D map, the atmosphere, the storyline, the responsiveness of Samus, the size of the game (it is HUGE)
Borrow it or rent it before buying. It would make a good first DS game, but don't expect a lot out of it. Heres hoping part 2 will kick ass.