It's held up pretty well, the sheer number of games available to play can be overwhelming at times but the classics ie The Simpsons, X-men, Final Fight, Turtles games, Metal Slug series, Wrestlefest, NBA Jam etc play just like the originals. The Fighting games actually play pretty well there's a million street fighter variance, king of fighters, Tekken and tons more. I am not a huge fighting game guy but if you are there's endless options. Just about all of the games has a save feature which is great, I started a Pokemon red game for the wife and we probably haven't played it in a year or close to it but I should be able to jump back into it today.
The downloadable game market is nice as well, was able to go on there and download SuperMario land 2 Six golden Coins from the Gameboy area and it was such a blast. They have a lot options and are constantly adding more, I think they also just re-organized the system and added more games. What's also great is they sell a knock off playstation controller that can plug into the system that you can use on games where the arcade board isn't as helpful ie the Mario games and a few others. A lot of the games arcade games like Simpsons, X-Men, Turtles and such you can have endless continues so its great if you have younger kids or have kids visits so they can die a million times but the game keeps progressing. I think they also jsut came out with a new system that isn't the arcade board but set up like console that has like 30K games on it. Still the arcade board is nice a feature and a necessity for some games.
Now there are some drawbacks, playing on a arcade style board it can be a little bit of trial and error to know which of the six buttons match up to what especially if your playing an emulator from a system where obviously what the game is saying to do isn't feasible (ie hit the Y button or right Trigger etc). Most of the games are fine as its usually the first few buttons that match up and most games don't need anywhere closed to the six you have but on a select few the buttons are spaced out a bit, NBA Jam if I recall was like this but you get the hang of it and the system offers you the option to re-order the buttons if you want ( I have not done this) or as I noted above you can simply buy a controller and play using that.
Most of the games I have played have felt and played just like I would suspect, some really well but as orangezoo noted I am sure there are some that might be an issue. Some of the console ones can very much feel like their age even with the emulator but again that's been the exception not the rule. Only other major issue is that its built I believe in South Korea, so some of the games have duplicates that are in Chinese or Japanese but 90% of them are the American or European version. This can be a bit frustrating though as every once in a while you see a game that's only in Japanese or Chinese or for example they have about 12 different Pokemon games but I had to download the American Red version as all the loaded ones were in Japanese or Chinese. Also sometimes the games are given weird names which I am not sure if its supposed to be a joke or done for some other purposes. For example you might see listed "shell bashing 3" and when you open it its Turtles in Time.
Still the pros are far and above the cons, its' great to have all of these games on one system that I can fire up at anytime when I feel like playing or need a nostalgia boost. The first time you play the Simpsons or X-men on it you will feel like its 1992 and your at an arcade. Let me know if you have any other questions I would be happy to answer.