I think Thunderbirds are extremely comfortable for playing on the couch. No upper horn to dig into your ribs.
A low end, bolt-on neck Thunderbird is far better than it has any right to be. I'm on my second one, and I like it right there along side more expensive Epis and far more expensive Gibsons. It's different in that the neck on a bolt-on Epi is much more like a Precision neck, only rounder and deeper. They do neck dive if you try to play them like a Fender, but if you hang them a little differently, they don't. I prefer a headstock up neck, which requires moving the strap button from where the upper horn would be if it had one, to the heel of the neck where it is on a Gibson or higher end Epi. Gibson Thunderbirds will hang wherever you put them, in my experience. They have less neck dive than the average Fender. Epiphones aren't quite as well balanced, but they are not the neck divers they are made out to be if you approach them as Thunderbirds, not something else.
And, just to reinforce what other people have said about Thunderbirds - you are either a Thunderbird player or you aren't. If you are, then nothing else works quite as well as that shape. If you aren't a Thunderbird player, then that shape won't work for you. I'm in the process of trying to figure out how to modify Thunderbirds to sound like all kinds of other things from thumpy to the more usual Thunderbird angry piano snarl.
One other undocumented feature that Thunderbirds have is attracting women. No other bass gets noticed. Thunderbirds do. I don't understand why, but I'm not complaining.
