Grand Rapids, MI
Great so far My wife and I recently switched from AT&T to Ting, and we've been satisfied, for the most part. Sprint's network is about as good as AT&T in our area, and we have the added bonus of roaming (voice and text only - no data) on Verizon.
The main benefit for us is cost - we went from spending over $60 per month at AT&T for two phones with voice only (except for the occasional $2/MB or $.20/text when we accidentally used those features); this was after a 23% IBM employee discount. With Ting, our bill should be under $50, and we're using texting and data as much as we'd like.
The one problem with their pricing structure is the price of data - it's fairly expensive if used in large quantities. For us, this isn't an issue - we normally have wifi access anyways; it's nice that we can get occasional data access without spending $20/phone/month.
From what I've seen, their customer service seems to be very good - their employees are accessible and forthright with information. Activating our phones (including one port) went smoothly, though, so I haven't needed to contact them.
I'm very hopeful that they continue to succeed. Their pricing model should be very sustainable, since it (I assume) closely reflects the actual costs they incur with Sprint.