I'll poo-poo some of the first three replies and support 65BlevJim,80fbody, and Diplomat360 in their replies.

Yes, bent bar suspension have some geometry issues, but they aren't deal killers, can be corrected, and everything you need to upgrade can be found at Firm Feel. Aside from t-bars, which do cost more, parts are about the same cost as A/B bodies. Additionally, it doesn't take much effort to modify the front to use Camaro shocks and the rear can be updated to use traditional Mopar springs and shocks thus saving $$ over the F body specific pieces. This also has the benefit of eliminating the iso-clamp suspension which is mush.

Correct, there are is no aftermarket support in sheetmetal and interior. Aside from the '76 models, which had issues out of the factory, they balance of late 77-80 versions are no more prone to rust than other cars of the era. Since their production numbers were pretty decent, there are still a lot of used sources out there. Obviously the west/southwest for metal and the midwest/east for interior sourcing.

There is a guy reproducing very good quality spoilers, flairs, and louvers for the R/T, Road Runner, Super Coupes, and Kit cars, so these aren't hard to find any more.

Entry level prices will be better than A and B bodies as well. With careful shopping, rust free standard F bodies can be found for $3k or less. R/T and Road Runners will run you around $5-8k, and Super Coupes and Kit cars are going to be $8-12k, generally speaking as there are always values to be found.

For performance, they respond to all the traditional performance upgrades like any other car. If emissions are a concern in your area, you can find some ways to work around that, but they obviously going to have some impact. They can swallow a big block, 383/400 are easiest to fit, but 440 can be made to fit as well. Depending on the emissions requirements of your area, you might even be able to make one of these swaps look factory. The neat thing about F bodies, they are as wide as a B body, but as long as an A body. This makes working on the engine somewhat easy, but also makes them a more compact driver.

Personally, I think they are fun to mess with. I've always fancied the idea of building one for a handling application as Alterkation does make a coil over conversion for them. Drop it low, flare the fenders and put some massive rollers under it with a T56 shifted, 408 small block. I've built several drag versions and it was a non-stop source of amusement to click off a low 12 second e.t. against a 15 second restored 68-72 big block car. Below is the last one I owned, although I've had several over the years. Notice the lack of rust in the lower quarters. A big plus for later model versions.