As requested a little while ago, I will try to share my experience now more than one year after I asked the original question. We decided to use Windriver, but so far we have only written a driver for Windows CE 6.0, so I cannot comment on the cross-platform support.
Using Windriver on Windows CE 6.0 has both advantages and disadvantages. It means all our driver code is now in library, so it has been easier to develop and debug (compared to a standard driver that requires Platform Builder). So from a development point of view it has been nice. Performance has also been fine. There were some overhead in the beginning learning the Windriver API and how to use it, especially with DMA and interrupts, but I don’t think it was worse than learning the raw Windows CE 6.0 PCI API.
The only real disadvantage I can think about is that a “real” driver is easier to share between multiple processes than the library we have created using Windriver. In our application (embedded system with one process) it is not really a problem, but it is harder to create debug/development utilities that operate on the hardware behind the back of the main process. We have used that approach for testing/debugging on other platforms but it is a bit more complicated to do here.
To sum everything up, I think we made the right choice and I am happy that we have the ability to port our “driver” to Windows Desktop with (hopefully) very little effort when we need it.