I like Unearthed Arcana, with some reservations. I really don't think (at time of writing), 25GBP is a competitive price for a PDF, especially for what is now an "obsolete" system.
For those who don't know, this is a collection of alternative rules - some you can add to an existing D&D 3.5 game, some replace extant rules. Some come from other d20 systems (such as the Wound/Vitality system from the Revised Star Wars rules), some are attempts to capture elements of fantasy literature in the D&D rules (such as Incantations).
It's really a grab-bag of ideas, pretty much all mechanics with just enough flavour to fire the imagination (which is better than the pages of useless fluff that accompanies many WOTC D&D products). You wouldn't use all the ideas in your game - you couldn't use all the ideas as some are mutually exclusive. Some concepts are good, but bland in execution (Bloodline traits and the racial variants, for example. Neither of these really award any abilities outside the norm).
However, most, if not all, of the content of this book is available online at the various SRD websites. I bought it in case there were some sneaky non-OGL parts. There aren't. So, if you are going for an electronic version you probably don't need to buy this, even though it does contain some interesting stuff.
[3 of 5 Stars!]