Ok, I got to try out my HM+rPi on a low-slow cook over the holiday break and am finally getting around to posting some results.
This was using the Nov 2012 firmware.
First, I was amazed at the HM's ability to regulate pit temperature.  
From my photo below, you will notice I jumped right to extreme testing to make temp regulation difficult:
a.  It was cold in Dallas
b.  My gasket was fried and leaking smoke (and heat) like a sieve
Even with this, pit temperature was rarely more than 2-3 degrees off target.
However, there were a few issues along the way.
1.  I lost 2 of the 4 Maverick ET-72 probes I ordered.  One died within the first hour of testing, the second during the cook and without warning.  When they died, I guess the connection opened or resistance jumped and they registered high temperature.  You can see in the screenshot below at 11:45am, my pit temperature appears to jumps from 225 to 375 in under 2 minutes.  I know the probes were dead because when I threw them on my Redi-Chek ET-72, the LCD just showed HHH.  Luckily I had extra probes on hand, so lid goes open, probe is replaced, fan kicks on full, and I'm back in business.  Maverick replaced the probes without question when I told them what the Redi-Chek was showing.
2.  I've had multiple instances where the rPi WiFi connection goes offline.  When this happens, the connection does not return unless I intervene.  I've had some luck pulling the Edimax WiFi out and switching it to the other USB port.   But, when I reconnect to the browser, all the graph history is reset and it really appears like the rPi has rebooted.  I looked once after this happened and confirmed sys uptime had been reset.  But if my system is rebooting, then the soft-reboot isn't enough to reset the wifi and let it reconnect.   I am starting more testing now, but would appreciate some recommendations on how to capture logs to catch the WiFi going offline and system reboots.  
 
BTW, screenshots are easy on a Android phone or tablet using Power+VolDown buttons simultaneously.
Gasket has now been replaced and I've updated to the 20121224B openwrt-rpi image.
Thanks!
Eric