I signed up in April of 2010. My experience with WIND has been pretty great. Only a few minor hiccups.
First, I signed up knowing right off the bat that they didn't officially have coverage in my area (Tuscany in Calgary). That said, I still get a decent signal (2 bars) at home; even in my basement. It only gets dropped when I'm in my laundry room. My only disappointment here is that they still haven't officially expanded to my area yet. This is likely because the only cell tower in the neighborhood is owned by Telus, and they can't get their antennas on there (see: tower sharing disagreements).
Next problem was that US roaming did not work. It absolutely refused to connect to T-Mobile when I was in Texas. When I contacted WIND, they recommended replacing the SIM card. I went into a store, it took about 2 minutes, and roaming worked fine on my next trip and every subsequent trip. My original SIM was white with an orange WIND logo. New one is mostly black with a white WIND logo. Since then, I've actually picked up a T-mobile SIM, which gives me unlimited talk/text/data for $3/day. That worked out much better for me, as my trips are usually pretty short, and I can use my phone's GPS tracking to get around without paying data roaming fees.
I once got a $0.15 text message charge when I was most definitely in the WIND Home zone. I was in a building where coverage was a little spotty, so my phone must have switched over briefly.
Call quality used to suffer (that weird half-duplex thing), but that's been gone for a long time.
Other than those things, I haven't had any unacceptable problems. No dropped calls. No loss of data except when my phone was acting up, not WIND's network. Signal strength in the malls could be better. I will lose signal when I'm in a big store like Old Navy, but I'm never actually using the phone at that time, so it doesn't bother me. Customer service has been really good to me. I strongly recommend calling during normal business hours, especially in the morning. Even better, their online "Live Chat" customer service is great because they'll email you a copy of the conversation. When I switched from the Holiday Miracle Plan to the Irresistible Plan (no longer available to switch to), I had them confirm that the features were identical, since I like to occasionally call/text the US, and use tethering.
At a fraction of what a similar plan would cost from Rogers, Bell, or Telus, I'm willing to accept that it's not "perfect" and that they're improving and expanding. The great thing is, as they expand, my existing plan just gets better and better because I'll be able to use my phone all around the country without incurring so much as a penny in additional fees.
So yes, I'm buying into WIND's "potential" but it seems like they're growing steadily, so there's not a lot to worry about.