I started using Panda Cloud Antivirus again yesterday, after they came out with a new version. I had tried it before, and while it seemed to work okay I eventually went back to MS Security Essentials. I wanted to see what had changed with Panda and if it worked with my secondary layer of protection, PCTools Threatfire. It installed quickly, ran an unobtrusive scan and then stayed out of the way. I did a quick test with a standard EICAR file to make sure it was working (it was) and will check on it as the days go by - otherwise, it seemed like the product was more more responsive than previously; looks good so far.
Interestingly, Aviria just announced a technical preview of Avira Protection Cloud - which at first blush, sounds a little hinky to me; it does not conduct deep scans, it only scans those folders that are likely to be infected. It also cannot detect or clean malware unless it has an Internet connection, unlike the Panda product which can work offline if needed. That could change when the Avira product is released, of course.
Panda has a well thought-out post on their site about concerns over using a cloud based AV, or at least their implementation of it, which is some sense is more of a hybrid approach than the Avira technical preview.