Some of you might remember a game called "The Fantasy Trip". This fantasy game was my first introduction to what would evolve into Gurps. I loved TFT, and I love Gurps even more so. I've been playing Gurps so long, I've had to replace my rule book twice from use. My current copy's spine is being held together by duct tape and is due to be replaced (here's a tip, if you buy this book, buy the hardcover. It will last longer than the soft) I love it for what it is meant to be: at its best, a flexible engine that can be applied to any genre. At its worst, it can be heavily munchkinized.
Gurps has a point based character creation system. Players assign points to basic attributes (Strength, Intelligence, Dexterity and Health)which have a base line of 10. The lower the score, the worse the attribute and vice versa. These four "building blocks" serve as the basis for all skills. Skills are neatly arranged in an easy to use format. Aside from skills and attributes, points are assigned for advantages. Advantages are just what they sound like, those things which give a character an edge. Disadvantages and quirks can be taken to give the player more points to spend on attributes, skills and advantages. It is disadvantages and quirks which give personality to the characters. The list of skills, advantages and disadvantages in the basic book isn't exhaustive, but comes close. Quirks are left to the player's imagination.
Skill resolution is easy. Roll three d6. Matching the skill level or lower indicates a success. Going over will result in a failure. Rolling exceptionaly well (low) or exceptionaly bad (high) could result in either a critical success or critical failure. Crits add to the enjoyment of the game.
Rules for combat are easy in their simplest format; however, there is an advanced combat system to supplement the basic combat system. If the advanced combat system is used, expect long, drawn out battles. Steve Jackson has a rule for just about anything, and if used, they will slow down play. Another word of caution. Munchkins will take forever making a character as they min/max everything and tweak their points to get every last possible advantage.
Gurps offers a workable system for psionics (Gurps: Psionics is a worthy expansion to the "basic" system in the main rulebook) but magic is one of the game's weak points. Magic is built on the concept that easier spells must be learned before harder spells (with this I agree) and that casting spells is physically exhaustive. Spells cost fatigue points and this score is derived from the character's strength. Now this I have a problem with. I realize that there must be a controlling factor, but unless your mage is Arnold or has a butt load of mana stones (mana is the energy for spells)she isn't going to last long in a battle. Gurps: Magic and Gurps: Grimoire do not offer much to fix this problem. Most players of Gurps end up tweaking the magic system to suite their taste.
There isn't too much genre information in the main rulebook; however, the self-sufficient geek with plenty of time on his hands could get by with just the main rulebook. I'd recommend purchasing at least Compendium I & II along with Vehicles and Martial Arts (just because MA kicks butt).
Gurps is well supported. There are rules expansion books (Compendiums I & II, Vehicles, Psionics, Martial Arts, Magic). Gurps does offer world books ranging from Space, Horror, Supers and Fantasy. Some of these I like (Space is good, Horror isn't bad) some I do not (I've never been an IST or Yrrth fan) but all of them give enough information that the GM can adapt his or her favorite genre settings for play (most DnD worlds convert easily). In some cases this has been done for the GM. Gurps: Discworld for instance is based upon Terry Pratchett's Discworld novels. Gurps also has a number of their own world's (such as Cabal). Steve Jackson games have put out a number of "period" books such as Greece, Aztecs and Russia. Overall, there is a wealth of supplementary material offered.
The Skinny: The rules are usuable in any genre. There is very little genre information in the main rulebook. That is left to the creativity of the GM, or can be obtained by purchasing some of the vast supplementary books offered. Gurps offers a highly detailed character generation system. This can be bad, if you do not like long character creation. Skills resolution is simple. Combat comes in two forms: quick and dirty with no attention paid to realism, and long and detailed with lots of realism. The Magic system bites and is the only reason I won't give Gurps 5 stars. Overall, something for everyone.