I just purchased the new PartyQ (not the old one -- the new one that's been out for a couple weeks) -- and I love it. I've used it for three 5+ hour cooks on my WSM 18.5 and on my UDS. For all three cooks, the PartyQ held the temp steady at 325F (hot-n-fast brisket for all three cooks).
My only concern is the battery life. I've not changed the 4-AA batteries (total cook time so far about 15-18 hours give or take) but I ended up buying a rechargeable set of extra AA batteries in case the batts decide to give up mid-cook.
I'm cooking in fairly hot and humid weather -- 80-90F for all three cooks -- so I have yet to experience the PQ when it gets cold. The cold -- and the reviews about the old PQ konking out in the cold or not working -- were what stopped me from purchasing it. However, when I saw BBQ Guru had a new one out, I bit the bullet.
I use a wireless thermometer to monitor the meat temp. I've got a DigiQ DX2 and a CyberQ wifi, but I've found the PQ to be quick and easy to set up. No cords to mess with, no worry about power outlets. I suspect I'll use this in place of the DX2 when I'm using only one pit. (I thought I'd like the CyberQ -- but I found that while it was accurate, my current wifi setup is not close enough for the CyberQ to pick up a signal. Plus, I really dislike that I need to use three food probes and a single pit probe with the CyberQ. It's great when you need all three food probes, but when you're smoking ribs or a single pork butt, plugging in all the probes and using none or only one seemed like a tangle of wires. But that's just me.)
Literally, you just turn on the PQ, slide it in the adapter, and set your pit temp. I believe it stores the temp from the previous cook (not 100% sure about this -- but now that I think about it, I only adjusted the pit temp the first time). The new PQ feels pretty sturdy, too. The temp buttons are nice and firm, and the power switch is under the LCD display. Everything feels well put together and solid.
There's a single probe that alligator clips to the pit grate -- the same pit probe as all the other BBQ Guru controllers. The new PQ has a little LED that flashes red or green to indicate visually the fan going on and off. There's also a relatively simple menu that you can access by pressing the two front buttons. The adapter is the same as the all the other BBQ Guru controllers, so if your pits already have the adapters installed, you're good to go.
I like the new flexible "arm" that snakes up from the fan and holds the LCD temp display. The arm makes it very easy to adjust the PQ so it's visible virtually no matter where you've installed the adapter.
Overall, I'm really pleased. No real issues, no real complaints, and the temp accuracy on both the WSM 18.5 and UDS is spot-on. I had to invert the the charcoal door on my WSM in order to get increased airflow (to take it past 285+), but once the PQ had the temp at 320 or so, it kept it steady for the whole cook. No issues. No problems on my UDS at all. My UDS runs hot, so it's easy to get it up to 300+ and have it settle where I need it.
EDIT: BTW, I sealed the holes on my WSM damper with high-heat metallic tape from Home Depot. Works fine. I'm able to maintain any sort of smoking temp I need -- from 250 on up.