I have a fairly new TM - only about 10 days of camping in it so far. No leaks at all yet. I have no idea how people are able to say they can set up or tear down in the rain without getting the inside wet though - perhaps other people's rain isn't like mine. Of course I think I'd prefer a little rain to wet canvas any day, so the TM beats a pop-up there (think about mildew).
Disadvantages:
1) More moving parts to break. I expect a latch to break at some point, there's a lot of force getting concentrated in a few spots. But so far it has been trouble free.
2) You have to set it up. This goes for a pop-up too. Takes about 15 minutes to set up, including unhitching, levelling, etc. You can skip some of the steps in a bind (or in the rain) if you have to and set up much quicker in a pinch.
3) You have to close it down. This also goes for a pop-up. It takes me about twice as long to tear down as set up.
4) Not being able to access the sewer hose storage compartment (in bumper on new models) while closed down (you don't have to set things up, just lift the back shell a bit to get to it - but still a pain).
5) Not being able to fill water while TM is shut down
6) Those darn steps! If you close down with the steps out, you have to set back up to pull in the steps!
7) Cost is much higher than most travel trailers with similar ammenities but which don't fold down. I think it's worth it, but higher cost is a disadvantage.
8) Factory A/C is *very* loud. Fortunately I don't plan on using it more than 1-2 weeks per year, as it would drive me nuts. I really wish I had the dealer demonstrate how it sounded before I bought the unit - it might have made me consider other options - it's that loud. It doesn't make much cycling noise though so some might prefer it over the quieter furnace which cycles on and off.
9) Snow camping. While you'll be warm enough, I have no idea how to get the shells down easily in the snow. You really don't want the weight on top when you lower them, and snow can be quite heavy.
10) No access to anything except the electrical plug, propane tanks, and batteries while it's closed. There is no accessible storage - at all - when the unit is closed. You can't get to the fridge with it closed. Kids aren't going to be able to easily get to the toilet when in the middle the desert in Nevada - you'll have to pop the thing up and set up the bathroom walls.
11) Some of the ruggedized popups would handle some of the gravel and dirt roads in the mountains much better than I think the TM will. I try to stay to paved roads, but am willing to use well graded gravel roads occasionally. I wouldn't even think about trying to get the TM into some of the best tent camping areas in my area, even though I had no problem getting a Geo Metro into those areas when I was a student.
12) Watching the tire shop take the wheels off my TM to balance them (WHY doesn't the factory buy a wheel balancer and spend 5 minutes extra and just balance the tires???), I know there is no way at all I'd want to try to do that on the side of an interstate. I'm going to need a new rim if a tire goes on my TM, because I'm going to drive it to the next exit.
13) Single axle on all but the largest TMs, with tires speed rated to 65 MPH and loaded near capacity.
Things I like:
1) Lightweight compared to a 26 foot trailer (I have an '07 2619)
2) Low profile, does great towing in 60 MPH+ side-winds (yes, I've tried that already)
3) Seems safe when towing. I've never feared the trailer getting away from me, but I have a full size truck - others may differ if they don't have a big tow vehicle.
4) I like the bathroom. It isn't a class A motorhome bathroom with full tub and 50 gallon black water tank - it's a very small bathroom, with a recirculating toilet. But it is a dry bathroom (toilet doesn't get wet when showering), and much nicer than anything I saw in a pop-up.
5) Has provisions for truly dry camping - it's self contained. Some pop-ups have gray water drain out an opening, with no gray water tank. That means you have to carry a tote with you to drain into.
6) If you do dry camping (no hookups), the solar panel option (factory or a larger non-factory option if your dealer does it) is nice. I have the 80W factory option, and I can say it extends my camping time.
7) It's well made. Yes, lightweight materials which means it won't take a huge beating, but it's lightweight materials are much better choices than many travel trailers.
8) Well-enough insulated. I've already camped in snow with no problems getting cold. I do make my bed differently in the TM - two comforters, one on the materess, one on top of the sheets. The bottom comforter is to keep some of the bed chill out. I imagine a pop-up could be worse in cold weather.
Things to consider, that I didn't spend much time thinking about
:
Think about microwave vs. oven when you consider trailers. Microwave, without a generator, is useless away from hookups. But if you never will use the oven...
I'm already planning on adding an electric tongue jack. Cranking the tongue jack gets very old quickly. Fortunately this is a relatively easy thing for a do-it-yourselfer to add, and not that expensive - or so I say right now before I've done it!
I'll add: I'm really glad I bought the TM. You get a lot of space, and a lot of ammenities, in a small space. The tradeoffs I made in choosing the TM have been well worth it for me. I wouldn't recommend a TM if you really want a huge fifth wheel or Class A motor home - you probably won't be happy if that's what you want. Nor would I recommend it if you plan on driving 60 miles each way on bad dirt roads to get to your campsite. But if you plan on staying in semi-developed or fully developed campsites, I think it's about the best thing you can buy if you are comfortable with the floorplan and setup/takedown.