Although I can't add much to this thread (because when it comes down to it, the fares and schedules for your favorite flights will make your choice for you), I thought I'd add a different opinion about AA Gold to counter ET's bad experiences.

I've been both low- and mid-tier on most US airlines, including all of the ones listed in this thread. I've now dropped out of all statuses except AA Gold because I no longer fly for business.

IMHO, AA Gold is the best low-level status you can have, assuming you have no chance of making a mid-tier. If you can make a mid-tier, NW/CO and their liberal upgrades come into play. But as a low-level, I've found that AA provides good customer service both at airports and on the phone, as well as a "fair" shot at a limited number of upgrades. I don't obsess about upgrades much as long as AA has MRTC. I'd rather ride 2 out of 10 segments in F on AA than 5 out of 10 on NW without MRTC.

I've also gotten the awards I've wanted (including Europe and Hawaii) without much difficulty, provided I was flexible with routing and dates within a day or so. Both United and AA have been good to me as far as international and Hawaii awards have gone: again, I provide a little flexibility and let the agents at AwardPlanner do the legwork.

The only downside with AA (I'll agree with ET on this bit) is that they are playing 100% hardball as far as the rules go, and those rules are getting tigher every day. The "bogus" fees around award tickets are exactly that (bogus). I have dealt with some nasty agents on the phone with every airline - but I get the sense that some of them are actually nice people caught between hardball rules and lots of irate customers. (Put me on the phone trying to enforce those garbage fees with my job on the line, and I'd probably turn into a Grade A A-hole pretty fast.)

Again, the fares/routes/schedule will determine your airline of choice, and keep in mind that there are people here who love and hate every airline.