I was a long time t-mobile post-paid user, then switched to t-mobile pre-paid, and then recently switched to simple mobile and have been using it for about week.
So far, I've been really happy with the service. Went to a local dealer, got a sim for $20 and $40 for the first month. Transferred my number over in a few hours, and after following the instructions on setting up data and mms on the SM website, my LG Optimus T worked just as well as before. Since SM really uses T-mobile, everything is pretty much the same - process for setting up voice mails, etc.
SM's account website is best described as simple - the way I like it. It takes a few seconds to get your online account set up and have monthly recurring pay automatically debited from your credit card.
For $40/month, unlimited talk, text, and 3g data, and essentially the same cellular service as before, for me it's hard to beat.
Currently my only complaint is I haven't gotten my GPS to work, yet. From before I remember T-mobile had some sort of screwy thing built into the phone where you need to have their 3G data plan to use the GPS signal received by the phone. I think I need to find out how to change some setting to get it work with SM. Let me know if any of you know something about this....
I'd highly recommend you give SM a try if you have a t-mobile phone already and don't have a contract. If it sucks, just switch back after a month