My personal experiences aren't so great so far. My speed is slower than my old ASDL 1 provider, but this may be due to the SNR profile they've put me on (used to get 3.8Mb, now get 3.2Mb).
Second is that I can't sign up for their forum as it says my details aren't valid.
Third and a very big concern, is regarding the "StalkStalk" thing. After visiting my site, I'm followed shortly afterwards by their URL following servers.
Example server log excerpts below:
62.24.222.131 - - [25/Jan/2011:20:19:10 +0000] "GET / HTTP/1.0" 200 9373 "http://retrotracks.net/" "(compatible; MSIE 7.0; Windows NT 5.1; Trident/4.0; .NET CLR 2.0.50727; .NET CLR 3.0.4506.2152; .NET CLR ;
http://www.talktalk.co.uk/products/virus-alerts/)"
The most concerning thing about this isn't that they visit the sites you go to, it's that they visit the SPECIFIC URL you visited. So if you went to a secret URL that triggers a cron job for instance, they will also repeatedly load that URL themselves later on, causing lots of potential issues (I've checked my server logs to confirm this). Very worrying to be honest. Looks like I'm going to have to configure the firewall on the server to block access from any of their servers.