I'm quite a happy owner of both the SounBridge and the PhotoBridge. The two co-exist quite well for my purposes.
The SB is an excellent device for listening to music of just about any sort. I purchased it originally b/c it was one of the few devices that played DRM protected content but then I began to realize how beautifully it streams WMA lossless and now I've become quite the audio snob and have not purchased any more online content. I use Rhapsody to listen to new music and when I find something I like I go and buy the CD and rip it in lossless format. For me - this is perfect.
I used the PB primarily (about 95%) for watching my DVD library using Mplay. It works beautifully! IMO, the only improvement to be made is for Roku to update their firmware so that Mplay can utilize full menu support on DVDs. Once (if) this is accomplished the Photobridge will be a perfect success as far as I'm concerend - but again, I bought it purely for streaming my DVD library over my network.
I agree with dgburns that eventually someone will come out with one product that will do it all and do it all very well, but I personally think that we're two or more years out on that prospect. Me - I'm too impatient to wait for that and with a decent remote (I use the Harmony 880) it's no big deal to swtich between activities so having one box to do both the funtions of the SB and PB wouldn't give me anything more than what I've got today.
Like dgburns said - the open source aspect of Roku's products is a HUGE benefit as far as I'm concerned.
Long story short - IMO, if you want to stream DRM WMA files, then purchase a SB. It works beautifully. If you want a device to view photos and stream video over your network then purchase a PB. If you want one device to do it all - be preapred to wait a while. Maybe I'm wrong, maybe someone will come out with the perfect device within the next 6 months. But I've been following this market for quite a while and between the forums here and the AVS forums, I've learned that this market is still very young. There are a lot of devices that do a lot of things - but they all have one thing in common. Every one of them has it's short comings. I think the biggest driver for this is the legality of streaming digital content.
Ok - I'll stop rambling for now!