EffiProz is an interesting database and it seems like they have made some very good strategic moves. For instance, Silverlight doesn’t support System.Data so EffiProz went and created their own class library that mimic's an ADO.NET provider since ADO.NET Providers aren’t supported under Silverlight. The same holds true for Windows Phone 7, EffiProz has a data provider for that as well.
We’ve had our eye on EffiProz ever since the initial problems with VistaDB were made known. What EffiProz is doing seems like a much wiser course of action for a smaller database engine as opposed to going after being able to comply with a very large Microsoft Entity Framework. Something we have been focusing on at EntitySpaces is being able to go wherever .NET can go, including Mono, which we support already. We will continue to support VistaDB as well (including new releases) and we already support the much improved Microsoft SQL CE 4.0 database.