Well I got this 4 days ago and am working on the fifth game in the series. Here are my thoughts on each: (Note: I've only played MMX1-3 before buying this collection)
MMX1: Classic and perfect. It seems to be a bit harder than I remember it being, but its still easy and quite fun. The best in the series imo.
MMX2: Also classic and perfectly emulated. The only thing that got screwed was the sound effects, everything seems 'muffled' or low.
MMX3: This one is a bit weird. Its the PSX version which included Cinema cutscenes throughout the game. Totally new sound effects and music for many of the stages. A bit different from the SNES version, but not enough so that you can't feel nostalgic.
MMX4: My first 'new' MMX game that I've played. At first I wasn't too keen on it, since I've been weaned on MMX1-3. But I took a liking to it quite fast, despite the simplistic level designs. There is significantly less stuff to find in the 8 boss stages, but that keeps the focus on the straight and narrow for the entire game. Having the option to play either X or Zero (or hard mode as I like to call it) is a nice touch. I beat Sigma with X but having a hell of a time with Zero, still trying though! Is it me or are the Maverick bosses in X4 harder than any of the others thus far?
MMX5: This is where I can clearly see the series take a sharp downfall. The whole time element, boss leveling up, get parts at lower hours to countdown from bosses, heart tanks only applying to the character who picks it up, making parts for your armors. It was all so confusing and felt.....weird. That and the levels were quite brutal and cheap with TONS of instant death spike traps or 1-hit kills. Also not liking the end where you fight all the bosses all over again with their life meters tripled, effectively drawing out the final level to a slow crawl. Furthermore, it seems the difficulty is unbalanced, where the levels would be more dangerous than their bosses! That and there is simply too much crap that is possible to obtain in this game that you will no doubt miss a lot of stuff the first time around. I will admit however, I thouroughly enjoyed fighting the rock monster and Rangda Bangda in their new forms. Still can't beat the final form of Sigma yet though.
MMX6: I've barely tried it, but this Nightmare system is quite....awful. Sometimes it doesn't effect things too much, but other times, it is damn annoying. Furthermore, it's like there are spikes wherever I go! Haven't got too far, but not really liking it thus far.
RUSH and BASS Racing: Haven't tried it yet, but I'm sure it'll be better than the last two games mentioned.
Overall, 4 our of six games ain't bad. Its still worth the money. what are your thoughts?