I'm using a program called "iFunBox", which is available for both PC and Mac. You just plug your iPad or iPhone into your computer, launch the program, and select "User Applications" on the left under "NeoRebel's iPad" (for example). Then, all of your apps show up in the main window. Double-click "Gridlee", then "Documents", then "roms" and just drag in whatever roms you want, zipped. If you want to run Neo games, you have to put the NeoGeo.zip bios file in there with the roms, and I had a heck of a time finding a bios file that worked.
I only have an iPad 2, so maybe with the faster processors of the newer models, things run better. For me, early Neo shooters run well. Andro Dunos, Zed Blade, Alpha Mission II, Aero Fighters II and 3. Games like Pulstar and Metal Slug X have slowdown, but I don't know how much of that is emulation and how much is just the way the game is. Windjammers and Mag Lord were both fine. Certainly any games from the 80's (talking about MAME games in general) should be fine. Early 90's are hit-or-miss, and late 90's and beyond are going to run slow. The only non-Neo game I've played so far is the original Strikers 1945, and it runs fine.
If you get an iCade, or better in my opinion, the iCade core (either way you have to mod it to upgrade the controls), then the iPad is an awesome MAME machine because of the high quality screen, and because you can easily rotate it to play games in TATE mode. Of course the same holds true of Android-based tablets, so I guess what I am saying is that tablets make great MAME machines.
A couple of suggestions would be to hit up the settings page and de-select "smoothed image" and select "scanline effect" in both the portrait and landscape sections. And if you do use an external controller, select "full screen" in both sections to remove the on-screen controls.
Chris