If I'm looking at building a game out of middleware, Unity seems like the best bet... and there are a number of Unity solutions promising MMO functionality for fairly cheap (between 50 and several hundred bucks, depending on the size of the license). Has anyone actually tried using them? Are any of them robust enough to really base an indie game on?
To be serious, I haven't used Unity, but I've followed some projects using it and most of them say nice things when they aren't actually chasing bugs in it. I'd suggest checking middleware vendors for projects using them (anybody that doesn't want to tell you, that's a huge red flag), and then maybe trying to contact those users directly. Whatever Project Gorgon is using seems to work pretty well, but I have no idea what it is (only that they did use something for their networking/client-server synchronization).
--Jello Biafra: "If evolution is outlawed, only outlaws will evolve."
Short answer: each possible solution needs to be compared against your needs.
I've been doing Unity development for about 3 years now. Built a multi-player fps with TNet. I haven't done a survey of the available solutions in a while. I recall looking at Photon and maybe Raknet integration when we started the multi-player project. The conclusions I came to a couple years ago were that the quality and ease of use varied wildly across the different offerings.
I ended up building persistent storage, authentication, and lobby processes as they weren't in TNet at the time. Luckily we had a source license on TNet and a good working relationship with the author, because we ended up making significant modifications to how games launch and player authentication. Oh, we also started with TNet at a very early release level so it may be much easier to work with at this point.