I used to use kaillera all the time and if you had a decent computer/connection and u knew how to punch port holes through your firewall there wasn't much lag at all for the games that I liked to play (tmnt, simpsons, street fighter, etc.) It wasn't just for MAME either, kaillera worked quite well for NES, SNES, and Genesis as well. But I think the biggest advantage to kaillera was when I would play through different floors on the same network, like a system link. If you wanted to have a good game of the Simpsons, had two cabinets, and wanted to give the players more space, kaillera would do that. Kaillera even worked well for a select few games of N64 like mario kart.
I will say kaillera wasn't exactly what you would call "user-friendly", it took a bit of finagling to understand the system link, some servers weren't as reliable as others, and it was a bit buggy at times, etc. I would definitely like to see an easier version of netplay in MAME, but who ever decides to do it is sure to have their work cut out for them.