I have done a total of four weeks volunteering with Vitalise, two at Churchtown (which they don't have anymore), one at Netley and one at Sandpipers. It is a great experience, but very tiring. It also takes a couple of days to settle in, each time I went I wanted to leave for the first couple of days, but once it got to Monday it did get better.
It is hard work, so make sure you take your breaks because you will need them. I also found that it was important for me to get away for five minutes, even just locking myself in the toilet, a few times during the week when things started to get on top of me a bit - spending so much time with the same people can sometimes get annoying, however nice everyone is.