I didn't like it because it was too much of an FPS compared to previous Metroid games. I loved Prime/Prime 2 and Super Metroid because of their exploration elements.
I don't think Prime 3 was as inspired as the first two - level design was linear, the plot was repetitive with American voice acting I disliked, the art direction was poor (everything looked the same, same problem the Conduit had) and it only had one good piece of music [Skytown] when the previous two games had lots of music I liked.
The controls were well done but the lock-on wasn't good enough, especially on bosses (kept missing).
If you want a generic 'space marine' FPS, there are tens of other franchises where you can get it. There is only one Metroid franchise and I don't want to see it taken over by that genre. Even Other M looks too focused on the action rather than the exploration for me, though I'll wait until the reviews of course.
