Looking at games on the shelf, one thing I noticed was an entire shelf full of GURPS books. I started playing GURPS in late 1988 while I was in college and had a new group of friends who played a lot of non-TSR games. My first experience with GURPS was when we switched a then-running Fantasy Hero campaign over to GURPS when we changed GM's. I liked it and went and bought the rulebook right away. We later played some GURPS Mechwarrior and several iterations of fantasy. My best friend picked it up too and ran a few fantasy campaigns and a space campaign for our other group too over the years. Being a longtime Traveller player too I picked up most of the GURPS Traveller books as well, though I've never run a game of it.
I like GURPS a lot and think it handles realistic levels of game quite well. It does get a little strange at the supers/ultra-tech end of things though, where you might be rolling 20d6 for laser rifle damage, but at the gritty fantasy/Traveller/ Mechwarrior end of things where you are basically a normal human doing fairly normal things plus maybe magic and psi, it works very well and is pretty easy for the new player to pick up. It's pretty easy to adapt/convert things as well.
I was a little sad when they went to a 4th edition in 2004 because my 3rd edition rulebook had been good for 15 years and I was sorry to see the compatibility break, but after reading 4th ed it's not all that different, really. I have most of the 4th ed books now though I have never run or played GURPS 4 - thanks to an amazing deal on eBay. All of the GURPS books are excellent resources on whatever subject they cover - my brief Classical Age/ Arcana Evolved camapign relied heavily on my GURPS Greece and Egypt books.
The most fun I had playing it was when a friend ran an anything goes time travel campaign so I used a combination of Ultra-Tech and Supers to make a hi-tech intelligent car that was invulnerable to most forms of damage - the Baron Industries Five Fifty, or B.I.F.F. We only played a few sessions but it was fun and the sci-fi runs were pretty straightforward but the old west adventure was challenging in many new ways.
All that said it's never really taken off with my main group of players. Some of that is that we were big into Hero system first, so if we're going generic or dimensional hopping or time traveling or super heroing then Hero is our first choice. Some flavor of D&D is always out first choice for fantasy, and Warhammer makes a strong case there too. For Traveller the GURPS books capture the default universe very well but the system is so different from classic Traveller mechanics that it feels off - MegaTraveller was my system of choice in the past but that has been replaced by the new Mongoose Traveller, which is simply great.
So I think the problem with GURPS in my groups over the years is that it's been at best a 2nd or 3rd choice for systems and doesn't have any compelling settings of its own to make it a 1st choice. Banestorm is a blast for history buffs (if you are one check it out) but most of my players are not that into it. Besides that one I can't think of a supported original setting for GURPS comparable to the Forgotten Realms, the Third Imperium, Deadlands, Warhammer's Old World, or the World of Darkness and I think that keeps it in the "tinkerer's system" niche rather than a full-fledged AAA system of its own.
As for me, if someone I knew wanted to run a campaign in it, I'd sign up for it in a minute, but I'm not going to start out a bunch of totally new RP'ers with it.