It's a bit early to tell, I haven't invested in .mobi but I think many have expected it to be better than it is now.
.Mobi isn't going to be all that successful because most companies are using either a sub domain or a /directory method for re-directing to their website's mobile version. The TLD's have gotten out of control. Next thing you know they'll try to push .co.mobi, .net.mobi, etc. on us. Just another TLD companies will have to worry about protecting from cyberpunks.
I think mobi won't be as big as .net. But looking for some sold domains for .mobi, you would have a second thought
the big guys are connected with .mobi .. they will make it big !! Its not as big as .net now .. but its making money 700,000 mobi domains are taken
To me the extension seems pointless, i just use useragent detection and send mobile devices to wap.domain.com