If you've never played a Metroid Prime before, there's been no better time to start. If you have, it's still worth it.
Of course, there are some minor issues: 1) Prime 2's multiplayer is still single console split-screen only: no wi-fi upgrade. 2) You still have to receive Friend Vouchers from a friend to get the green tokens, which are essential to getting some of the bonuses in Prime 3. Which leads us to my final grippe: Trilogy is incompatible with the credits, vouchers, and save files from the original Prime 3. That means if you been collecting credits in the original, they'd have to be collected all over again. In addition, your friends would have to send you vouchers collected in Trilogy's P3 for you to get the green credits, and vice-versa. On the plus side, you no longer need Metroid Fusion+GBA+link cable to get the Fusion Suit: just beat Prime 1 & buy it with the collected credits.
In summary, while there are still some improvements that COULD have been made, this new look on the Prime Trilogy is still stellar, and thus a must-have for anyone who considers themselves an action gamer.