Lets, get this straight: nothing can do what a tivo can, although it has to be admitted that sky+ can do some things a tivo can't. The Pace twin has a tiny hard drive, a useless EPG and is still buggy. The Humax doesn't even let you watch a recorded program while recording another.
A tivo has 4weeks of EPG data; you can set up season passes to record your favourite shows, wishlists to find programmes with your favourite actors, directors, subjects etc. It will automatically record stuff it thinks you might like and is intelligent about resolving conflicts when two programs clash. It does the pause live tv thing too.
It is hackable and upgradable in the extreme: you could have one with 2 x300GB hard drives if you desired. You can add it to your computer network and drive it from a web browser, even over the Internet. You can extract programs from its hard drive and burn them to dvd on your PC. You can even stream stuff directly from its hard drive to windows media player. There's loads of cool hacks like "endpad" which will start recording early and finish recording late if and only if it won't affect the recording of other programmes.
It works with sky, freeview, cable and analogue. Some say the the picture quality is not as good as sky+, but you can up the resolution and bitrates of recordings to dvd standard and you'd be hard pushed to tell the difference between live and recorded.
There are some good deals on ebay.co.uk. There's one guy who is selling a batch of new, unused units with a 1 year guarantee for £209 including delivery. See
www.tivocommunity.com for more info on what tivo can do.