It's not a textbook per say given the lack of maths in the quantities you'd find in a textbook but it's pretty good for understanding the conceptual features of more advanced material. I couldn't get my head around the notions of a fibre bundle from a textbook but Penrose does a number of diagrams and mroe wordy explainations and that helped a lot.
If you really like maths and realise it's not a 'casual read' then it's worth the £10 or so it costs.
I really like this ambitious book, but, as AlphaNumeric says, there isn't enought detail to learn stuff at a working level. Some may find the mathematics to be quite abstract, which, combined with the lack of detail, may cause frustration. If you find you're getting bogged down on some parts, just press on. Also, you don't have to read all of the first n-1 chapters to read chapter n. Read what interests you.
Penrose presents some standard results from unusual (and, according to him, uncontroversial) perspectives. On somer issues, however, Penrose holds views that are very controversial, and he takes care to label them as such.
All in all, one of the most fascinating books that I've read.