.com .net and .org are in a league of their own
.us had potential but doesn't seem to be doing so hot
.info and .tv could become huge given the right circumstances
there is no need for .mobi. the site could just determine you are on a mobile phone and redirect you to the proper html and coding. it's just another way to make butt loads of money from people trying to get in on the Internet Real Estate Game.
Stick with your top 3 levels unless you get an AMAZING keyword or you're doing arbitrage then it hardly matters.
you're never gonna be able to grab those domains as quickly as these guys do
they're registrars, firing 1000 queries a second to snap those top domains
if you're just a regular guy, don't even bother