When I played around with the PS3 and DLNA servers for the Mac, the experience was inconsistent and it sounds like it's not bulletproof reliable for others either.
*nod* *nod* Early DLNA software was flakey.
Also depends on what media you want to play. If they are home encoded, or downloaded somewhere from the net, then you'll need to know what you're doing.
Videos straight from digital cameras should be fine. Those that are compliant with MPEG4 standard (Main profile level 4.1 and below) are usually ok. Recent DLNA software seems robust too.
The DLNA link between iOS devices and PS3 works on first try, and snappy too.
I haven't bought an AppleTV so can't comment if AirPlay is more reliable, though people say when it works, people would pull out their iPhones and send media out to the big screen at parties and people would gather around.
I don't have AppleTV either. People on iOS typically play licensed or application videos. Their job should be easier compared to playing arbitrary media from elsewhere. e.g., iPad can't play 1080p videos.
I'm sure I'm not alone to want to play iOS content elsewhere though. ;-)