Generally seems to work fine, however...
I mostly use play.com and amazon.co.uk (often the best prices/section). It seems that it will compare Play prices to Amazon prices, but not vice versa. I prefer to browse on Amazon, so this means I have to manually check Play prices each time, which kinda defies the point.
Also, I would rather it allowed you to switch off automatic checking and let you just check using the status bar button on demand. I find the automatic checking of each page unnecessary most of the time, and that it slightly slows down browsing.
If the above are not problems for you then this seems like a good tool, and it definitely a good idea. Unfortunately, the way it functions at the moment it isn't for me.