Our company currently uses a Sonicwall NSA 4500. We switched to that from a Watchguard X1000. (a precursor to the XTM model you have picked out) To be perfectly honest, I'm not happy with the switch. (and it was my call -- oops) The Sonicwall has a better (web) interface. It can make some fancy graphs and pie charts, and updates nicely in the browser. The Watchguard interface was definitely clunkier and was a thick client, but I find I prefer that.
The Watchguard also was a bit more explicit in what it was doing, and why. This made determining why packets were being dropped much easier. That being said, it might be a double edged sword, as it could easily be harder to parse for less technical admins? I've had several issues of the Sonicwall silently dropping connections, and only noting it when doing a packet capture through the appliance. Sonicwall support was then still unable to really determine what was going on, despite having drop codes from the appliance. (So, if anyone out there knows why my Sonicwall keeps randomly killing NetApp Snapmirror replication, please let me know!)
Back when we used Watchguard, (~two years ago) their support was atrocious. It was outsourced to India, and resulted in having to jump a language barrier on every single call. Usually the person taking down the initial ticket info didn't catch the nuances in the problem. Their sales guy claimed this was changing, but we didn't stick around to find out. Sonicwall support staff is all English speakers. However, as mentioned above, they can't seem to help with some of the harder issues I've brought up. Wait times with both companies has been quite painful.
I can't claim experience with Juniper or Cisco equipment in this role. But both those are very companies, as are a good number of their clients. Both WatchGuard and Sonicwall aim for the small/medium business market. So that may or may not be worth considering, depending on your company size.