In my 14 years or so here in LA I've bounced around between AT&T, Covad, and DSLExtreme. All pretty much the same in terms of customer service in my experience, but perhaps the more important differentiators are pricing and availability of service depending on which locale you're in.
It's sometimes worth changing ISPs every 2 years or so once your initial 12 or 24 month 'promo' period expires and if their retention department doesn't offer you a good deal.
Check out DSLreports.com for more info and resources too. Oh and if you do decide to go with AT&T let me know as I can 'refer' you, and we'll both get some swag.
Hope that helps.