I used a poly tank on my last FJ40 and am putting a new one in my currrent cruiser. I was very satisfied with the poly tank, I did not run a top of any kind so maybe that is why I never smelled any fumes.
The only problem I had initially was the sending unit sealing properly. I would recommend a good gasket sealer along with the the gasket to seal the sending unit. Also, do not over tighten and strip out the bolts, you need to just snug them. Be careful not to drip any sealer inside your tank, it will clog your fuel system. Doing this while it is out of the vehicle is a good idea. Also make sure and plug off any unneeded lines. I am smog exempt, so I only use the main fuel feeder and a breather line. All other connections are plugged.
The OEM tanks are heavier and rust out. If you notice, there are many new cars running poly tanks. I never noticed any collapsing of anykind during the summer months, even on 100 degree plus weather.
From what it sounds like on this thread, there are quite a few other unsatisfied customers... hopefully your experience is a positive one.