I'm still using Untangle for the majority our business clients that have UTMs...we still have one client using Endian..they've been using it since before Untangle hit the market.
I'm currently looking into Astaro "Red"...which is a special WAN product for businesses with small branch offices. Astaro is another UTM product that has been around for quite some time, very solid and mature product. However, their UTM product is not "free" for businesses, although they have a free basic product for small businesses. Most of their products are "pay for".
What I like about Astaro "Red"...is the easy to deploy and manage satellite office routers. They basically tunnel all traffic to mothership..and internet traffic gets cleansed by the main UTM appliance at mothership. Centrally managed, and pretty much zero configuration to deploy the remote units.
Due to the size of the client I'm looking it deploying this at, with 1 and 2 PC branch offices in tiny rooms at courthouses and police stations...I really don't have the room for a larger UTM box like Untangle.