Bonk's biggest problem is the lack of the turbo slider that the TG16/PCE controller had. It's probably just my own over-reliance on it to begin with, but the boxer and twin-ninja-dino bosses without it, as ferricide said, are 20x harder than they were in the original. Other than that it's a pretty slick remake, barring some strange texture work in places.
Adventure Island feels the most solidly produced, looks the best of all 4, and plays exactly like the original. If you dig the game at all it's definitely worth the low cost.
Cubic Lode Runner is still my favorite of the 4, and like Bobby said is one of the most ignored, if not outright maligned (ima lookin' at you, toastyfrog) of the series.
Star Soldier is the last one I have yet to buy, mostly because I can't explode watermelons with my hummingbird-like index finger, so I don't feel like I'll get as much out of it.
All of'm have cool unlockables, too. Play-Asia's got all but Star Soldier for the $20-25 in both regular and Za Besto flavors, and I wouldn't stop anyone considering picking them up for that, especially if they've got even just an ounce of love in their heart for Hudson.