I've been using Web Matrix for a pro bono project and for this kind of small web site that needs a little bit of dynamics it's very well suited. It's extremely fast to get a data-driven site up and running with beautiful razor syntax and layout templates.
The biggest downside I've seen so far is the very primitive database support; there are arbitrary restrictions that make the GUI management of the database schema very prohibitive and counter-productive. Simple things like renaming columns and deleting colums with relations or adding columns with non-nullability is often not possible.
The second downside is the lack of Intellisense.
But apart from that it's a very nice productive tool for light-weight tasks. It's taken a few hours to setup an existing site with a layout template and a few more hours to get a basic database schema and order management system going. So 1-2 days worth of work is not bad for a dynamic website I think. A weekend's worth of work and it's done.
The DB schema admin. is very annoyingly restrictive though. T-SQL is more useful here.