As someone who works for a reseller, we were faced with the same issue when McAfee stopped selling the SnapGear/UTM Firewalls.
While we still sell Firewall Enterprise, we went for the SonicWALL TZ series to replace the hole left by SnapGear and I am very impressed with them. They aren't strict equals, but in many areas I would go as far to say that the TZs are superior to SGs. But in other areas they are different. Many SG owners use the PPTP service for remote user access and, of course, most client operating systems include PPTP client functionality - meaning no additional cost. However the SonicWALL devices don't have PPTP. There's IPSec and SSL VPN instead.
General advice is to ignore the 105 & 205 appliances as they generally aren't consiered to be that powerful (but better than the 100 & 200 models they replaced last year) and don't include all the layer-7 application awareness. However the 215 is a more than capable unit as a replacement for the SG56x series.
Hope that helps.
Direct via email, and their reseller, are suggesting the TZ205. I'll have a better look at the 215, and ask about the layer-7 application awareness. Maybe I can make them think I know what's going on.
I'm also a little (lot) frustrated with their 'reseller'. I told them straight up what I'm trying to do.
They want to charge me $500 to ship the device in here, pre-configured.
I asked about my problem with the 3G hub, and Dyndns, and they said, "Oh, you'll have to sort that one out yourself."
I also hope that their forum members are as helpful as they are here. This forum has saved my butt more than once.