In a nutshell (and I responded at length directly to Ward) I never wanted to stop using DF, but I employed a consultant to code the core of our system while I was working at another client site, and he wanted to use RIA as it was an environment he was already familiar with.
In hindsight, given that he failed to produce anything we could use, we could have swapped back to DF when we restarted the project, but when the contractor we employed explained that he had been using RIA for over a year we agreed to stick with that (after all, it's his solution, so we had to make sure he was comfortable).
I don't think WCF RIA has any particular problems, and these two system are almost identical from a "Microsoft Twitter Client" project point of view, but once you start thinking about more complex data issues, per-entity security, and (something I consider very important) a way of providing 'unit of work' through synchronising data changes between Data Domain Context's without requerying the whole set from the database, DF becomes much more usable.
Add the disconnected data cache (makes mocking a doddle), and it starts to look very attractive.
The DF method of documentation using PDFs backed with example projects is an excellent idea and one that developers can easily relate to, and the quick responses in the forum are much better than posting a WCF RIA question on a news group where most of the responses are "Why do you want to do that? You should redesign your database...", or Googling to find someone somewhere blogging on the weird behaviour you're experiencing in RIA :)
Will we actually convert our project back from WCF RIA to DF2010? Well I did the total conversion as a test in a day (including coming back to speed with DF after almost 12 months), so I know it is a relatively simple task at any time.
I'll continue experimenting with it in a branched build, and will probably toggle us back the moment we find something major that WCF RIA struggles with, but it is a retraining issue for our contractor, so I'm loath to interfere with hs creative flow at this stage :)
In summary, starting a new project I would definitely use and recommend DevForce2010.
I consider RIA to be Microsoft's solution for the general public, providing a safe, predicatable, and bottled solution.
I think of DevForce as being a programmers solution, matching all of RIA's features and simplicity, but scoped to include a more technical audience; addressing requirements beyond RIA's scope that most people don't realise exist until they are 95% of the way through their projects.
Cheers,
Jason