So this is a review a long time in the making. Partially because I had to actually play it, but also because this game is huge. I can see why people enjoy it, but anyone who claims this is the "perfect system" (and yes I've seen those exact claims) are really just fooling themselves.
Not a System
First of all I'm going to be blunt with what I both enjoy and hate about GURPS. If you come into this game thinking it is a system you are going to screw yourself. Like Fate this is more of a toolkit to make the system you want than actually a system itself. Yes there is the "standard" way to play GURPS, but even in the Basic book there are so many options and switches in every aspect of the game that even determining what is "standard" isn't always clear. While I get that there is a level of modularity needed for any genre neutral game there is a difference between a core set of rules with little modifiers for flavor and rebuilding the entire system up with only the fact that you roll below a target number staying the same. In all honesty most of this wouldn't be an issue if it weren't for the next hurdle when it comes to learning GURPS.
Get It Organized
The basic book feels like it is still about three drafts of formatting before it should have gone to print. While reference page numbers are common and nice the fact that ALL advantages are in a single 50 odd page section with no separation of what it mundane, supernatural, and even super heroic other than small coded images in the names. This means unless you want to have every advantage available (something that you REALLY don't want to do) you have to go through the entire section, pick out, and basically organize all the advantages to figure out what should and shouldn't be in your game. This is the same with skills to which I'm not even going to give you a number because there are too damn many. While I like how they handle not having ranks in a skill (I'll cover that later) it's such a massive list that players end up feeling like they aren't going to be capable at 99% of what comes up and GMs are going to constantly worry they are choosing the wrong skill. Yes a GM could go in and easily pare down the skill list to the skills he would want to use for his game, but again this is organizing areas that the boo could have easily done in advance. I enjoy the options between a simple combat system in the Characters book and a more in depth advanced system in the Worlds book, but character creation is a joke. And this isn't even mentioning the issues like unclear rules for things like size modifiers, status, and so on that were all lumped into a single section.
Worlds to Play In
Part of the problem in my opinion is the choice of their standard setting for the game. Making use of the flexibility of GURPS the writers decided to make the standard setting be one of world hopping to allow any character idea come together and work. I actually like the setting concept, but the problem come in when they mention things like making a wild west gun slinger and afantazy wizard, but no tips on what advantages or skills will go with what. The tech level system helps a bit, but that it mostly for gear and not the actually creation of characters. A multi genre hopping cross univers game is fun, but one of those things for seasoned gamers and a handful even in systems that help guide what rules to use for what setting. I feel like this is on purpose as if a GM wants to run a wild west game they can then pick up a wild west setting book or have to basically write their own book to make things work.
One Second at a Time
This might be my favorite aspect of everything GURPS brings to the table. Round intervals are 1 Second each. I've always felt time intervals were arbitrary with most being seemingly random amounts of time, but at 1 second a round everything simply breaks down easier. Yes it means players will do less each turn, but this also removes that aspect of one player getting to do 5 seconds of work and everyone else basically being frozen until it's their turn. No more breaking things down into how many turns was in X amount of time, its just that many seconds every time.
Roll Under Not Over
GURPS uses 3d6 with the mentality of your skill being a target number you have to roll under rather than a bonus to a DC. This combines possibly my favorite methods or number generation as rolling under means skill is more represented as your ability to do something rather than a flat bonus that can eventually make all but the highest of DCs impossible to fail. I'm not one of those that looks to roll for everything, but if there's never a chance to fail when it's dramatically appropriate then it jsut becomes harder and harder to build tension. As for the 3d6 this creates a curve to the probability of success meaning low skills are incredibly bad, but it's easily to become competence, and more importantly after a certain point your generate diminishing returns on adding higher numbers. I don't ever want to see my players fail, but if there's no chance of it then why even play rather than stand around and say how awesome your character is. This creates possibly the best balance of feeling competent and having a chance for things to go wrong.
So this is where that modularity that can be a mess turns into the games greatest asset. Once you DO get a handle on the game and more importantly know how much of it you want to use this game becomes just as loose or in depth as YOU want it to be. The catch being that obviously everyone has to be playing at that level. there's some room for that one player that wants all the tactical rules while the rest can just ignore them, but in the end those are still rules the GM will have to know or look up if they are at the table. In the end this game can be striped down to bare bones and still run (look up GURPS Light and Ultra-Light) and that says a lot about the stability to the core system.
The Game You Want
In the end everyone who says GURPS is the best system out there are ones that made their idea of the best system out there using GURPS. This is why I call it a toolkit and not a system on its own. If you used every rule offered (even if you ignored the ones that contradict other rules) you would have a mess of a game. Sadly without proper direction in the books and how to pear down the rules this is closer to what many first games will turn into. That said, if you stick with it and take the time you COULD build a game you love. The question is if that's something you want to bother with. Alternatively there are tones of setting you can buy that do all that work for you and really I would say look at most of those as their own games since hating one doesn't mean you wouldn't love another. Personally my group is playing Monster Hunters and now that I'm in a setting that did most of the work for me I'm enjoying the system, but it also shows some of the basic book's bigger flaws. Especially in how the setting book breaks down what and how to use different skills in specific situations that will come up often which the basic book could have easily down to teach new players.
In the End
If you're will to put in the work GURPS can be a great system, but only if you manage it. Expect your first came to be wrong and not just missing a rule or two. Expect to have to scrap most of what you ruled was allowed after actually playing with the game. In the end this isn't a bad game, but should NEVER be a palyer's first step into gaming.