I've been proud (more or less) owner of a 2000 year model Coleman Sea Pines pop-up for about 7 years, and we use it several times a year. I really like them and think they're sort of a best-of-both worlds camping soluton. It has a king and a queen bed, decent seating, an indoor sink and stove, and AC/heat. it doesn't have a bathroom, but I'm okay with that.
Jayco makes a good trailer, and if I were to replave my 12' Coleman, a 1206 Jayco is what I would buy. When looking at use campers, there are several things to check - canvas condition was already mentioned. Roof condition is also REALLY important. Most Jaycos use an aluminum roof system, so check all the joints for peeling seals, missing caulk, etc. If for some reason it has a molded plastic or ABS roof, run away, but I think only Colemans (like mine) had them. Mine is currently cracking from sun exposure. Leaks here can be a nightmare, and check around the roof mounted AC for leaks. Make sure the AC and fridge work. Check the electric brakes for function if equipped. Look for signs of mildew or mold anywhere in the camper, signifying a leak or poor care. If you are really sloppy about stowing the camper wet after a rainstorm, the mildew can be horrid. Same for a roof leak. If the camper has been stored inside or under a shelter out of the sun, that's a HUGE plus.
As for camping - In the hottest weather, they usually stay a little on the warm side, even with the AC running full tilt. Staying in shaded sites when possible helps, as does pinning a shiny emergency blanket on the outside of the canvas over the bunks. there's just no insulation there. When it's really cold, (like 20F, I'm in GA after all) you'll want a small electric space heater to supplement the onboard, it's only good to about 35F. Make sure whatever circuit you run the space heater on can handle it, I actually rewired mine with a small home-grade breaker box. If it doesn't have a power roof lift, go to Harbor Freight and spend $50 on their 1/2" High torque, low speed drill. Buy an extra hand crank and cut off the socket to chuck in the drill. Instant power lift, but don't over do it, you can destroy the lift system if you lift it too far.
Why do I think it's the best of both worlds? comfy beds, AC, and still feels like a tent sleeping under the canvas. Plus, when the weather's nice you can roll down all the plastic windows and you have a giant screened gazebo. And the engineering in how they work is just the coolest.
For more info than you care to know, visit popupportal.com
BTW, used camper values are E36 M3e on a popup, so check the craigslist value on any trade he offers. Don't give away too much for it.