I have it. I thought I'd try it because I work away from home and use my laptop with a connection through a mobile phone on GPRS. It's good for that because it compresses the data so I can surf the net without paying a fortune in GPRS charges.
I had problems for a while with mail not arriving until a couple of days after I sent it but I'm not sure if that was Onspeed or not. A mate of mine reckoned it was going through a ridiculously convoluted route to get anywhere (don't ask me - I only use computers. I know nowt about the workings of them!). Seems alright now though.
It does seem to have to look for the Onspeed server quite a lot i.e. quite often in between pages if you have been on one page for a few minutes so if it is faster (and I couldn't really say on dial-up because I don't really use it at home) it's also a bit of a pain because it is "trying to connect" a lot of the time instead of getting on with the job in hand,