In addition to what has already been mentioned, might I add a little something about their IPS and content filtering.
I don't know what you have planned for the SonicWalls, but our gateway router into our building is an NSA 3500. We're a company of about 85 users. We have licenses for the Content Filtering System, Intrusion Prevention, and the Application Flow Monitor, which are all superb. I can check how much of our bandwidth is going to Pandora or youtube, I can see what files are leaving the building. If I pull up the logs and I see that a user is using bittorrent, I can kill the session and block future bittorrent traffic in two clicks. For bandwidth management and security, it's great.
We also have SonicWalls in our remote shipping office and at our colocation where our production servers reside. We have VPN tunnels (site-to-site) between the three sites and it's ridiculously easy to configure. At our colocation is where we have two nsa 3500's in an HA pair. We did a few failover tests when we first set them up and we haven't had to worry about the pair since. We're also looking into licensing the HA pair with Intrusion Prevention, which will detect brute force attacks, sql injection attempts, etc., with what they call their "deep packet inspection" engine.
I've been more than satisfied with our SonicWalls.
Some things to look out for, however, is the licensing. Sometimes it feels like they're taking every penny you have each time you add a feature. I believe the only licensing you have to worry about with HA is if you want it to be a stateful failover. If this is licensed, any connections to your primary sonicwall will be there and ready on the backup in the event of a failover and any existing connections won't be interrupted. I think that's it though.