Get a lot of the progs I watch on the cable video on demand equivalent (which predated iPlayer by a long time) so I don't use iPlayer perse but it is similar thing. So depending on your philosophy I do/don't use these kinds of services. That and I'd get traffic shaped if I'd used iPlayer!
I like it in principle, but wish it worked a bit better. I've lost count of the number of times the player has frozen, or failed to resume after I paused something. Then I have to start viewing from the beginning as it won't let me jump forward. I hope they get these issues ironed out because it feels very unpolished compared to YouTube.
I used it today to watch the last Robin Hood (a good day to die). Downloaded the file for more flexibility than streaming which can lead to some of the problems touched on above. Tried to burn it to DVD so that we could watch it on the TV downstairs but was foiled by the dreaded DRM which I could remove using Tunebite but the resulting file was hideously jumpy and unwatchable. So ended up watching alone on the main PC upstairs. Apart from that it is a pretty good service.
What I don't get though is that I couldn't find anything under that Ts and Cs that mentioned that you had to be a license payer to download!
as a mac fiend i'm annoyed it's windoze ie only, but new macbook can be booted in windoze vista (and run faster than most pc notebooks) so at least i can use it that way. virgin's on demand stuff is easier, but i kind of like it anyway. a mac version is in the pipeline too.