A few problems set this back from being the must-have Sonic Collection.
Many people will complain that the emulation is off slightly, so the controls just aren't as tight as their original counterparts, but I've never personally experienced these problems. There's only a few gripes I have with it, but the problems are very noticeable, and in some cases, can ruin the experience.
First off, the games have been stretched to fit the aspect ratio of the DS. Because of this, the games don't look quite as good as they used to on the Genesis, and sometimes the camera will be a little jittery. It would have been nice to at least have the option to have the original aspect ratio, like how in Contra 4, you get the option to play with the original aspect ratio when you're playing the NES Contra.
The other biggest problem is the frame rate. For Sonic the Hedgehog and Sonic 3/Sonic & Knuckles, there will be times when the frame rate will drop and it may cause a few unfair deaths because you can't see perfectly what's going on. For whatever reason, Sonic 2 doesn't seem to have that problem. Blast processing maybe? Between Sonic the Hedgehog and Sonic 3, it's most noticeable on the original Sonic the Hedgehog, almost making the game as bad as Sonic the Hedgehog Genesis for the Gameboy Advance.
In regards to the collection itself, it's limited. I know it's a DS cartridge, but they could have put a few more games in, like the Game Gear or Master System games. Maybe even Sonic CD and/or Knuckles Chaotixs. They could have removed the picture gallery and put in another game instead. No one even looks at the gallery anyways.
The way the games work is very simple, and quitting a game couldn't be simpler. You get an option on the touch screen during gameplay that says "quit." You just press that, and you go back to the main menu. No load times or anything, it's actually a flawless system I quite enjoy.
Also, these are six of the greatest games ever created, let alone the six best Sonic games. Even with frame rate and aspect ration problems, they're all very enjoyable to play. The only one I don't enjoy playing is the original Sonic the Hedgehog, because it's pretty much just like playing the GBA title.
Overall, it does have a couple big problems, but they're still Sonic the Hedgehog, and for $20, it's a pretty good deal. If you can get the original Genesis games, I'd recommend getting those, but this isn't a bad deal at all.