I am already on my third h6315 in two weeks.  Yes, number THREE!  Here is the log:
I got number one on Aug 27, one day after its official release.  The T-Mobile store was out of stock initially and had to get 2 (and only 2) from another location.  I got one of them because I managed to convince the sales person to give me higher priority on the waiting list because I would be a new T-Mobile customer and I had already pre-registered with T-Mobile.com.  Immediately after getting the unit I experienced frequent lock ups.  The worst part is that the darn thing wouldn't connect to any WiFi.  On Aug 28 I brought the thing back to the store and true enough, it was defective - it wouldn't even connect to T-Mobile's own hotspot.  Unfortunately I couldn't exchange it then because there wasn't any stock in store.
On Sep 8 I called the store and was told that they just received twenty units.  Yippy!  I had the sales person take out one unit and test its WiFi for me.  Right after work I went to the store and happily exchanged unit number one for this spanking new number two.  The screen didn't feel as smooth compared to the first one when I used the stylus, but I thought I finally got a working $500 PDA-phone.  The happiness last less than two days when on Friday Sep 10 I started to realize that the phone wouldn't wake up on incoming calls.  Instead when I manually turn it on it would briefly flash a incoming-call message followed by a missed-a-call message.  I conducted some tests and verified that basically the phone part of the $500 gadget doesn't work.  Luckily this time the store still had some units remaining.
Getting very frustrated by then, I couldn't wait till after work to resolve the problem.  Instead I went down to the shop in the middle of the afternoon - hey, it's a Friday.  I got my number three, and that's the one I am still using.
So far I think this third unit is a keeper, but in the next few days I would still conduct whatever tests I can think of.  The funny part of this "journey" is that according to the sales person, I am the only customer at that store who had experienced problems with h6315s.  Oh lucky me.