TiVo is capable of far more than other DVR in the UK due to the excellence of its software design. This means it is in many ways superior than any current UK DVR and will probably remain so until 2011, despite not having any significant software updates since 2001!
In addition there are thousands of hacks available to further extend its functionality. There are quite a few books on the subject and literally 10,000+ programmers and hobbyists who regularly hack TiVo and millions of web pages devoted to it.
I think there are 4 or 5 million+ TiVo users worldwide compare to 1 million Sky+ users.
There are just 2 hacks for Sky+
AFAIK, versus thousands for TiVo.
The UK TiVo hardware is over 5 years old and 'obsolete', yet I will probably be still using TiVo in 2011 (albeit with a few terabytes of disk space)!
If you want better picture quality, get HiDef Sky. If you want a cheaper DVR get a Freeview DVR with twin tuners or a DVD HD recorder. If you have a hefty Sky sub, get Sky+.
If you want the best PVR experience IMHO, get two or more TiVos, with one STB feeding each one.