I've got the Enterprise C - for the most part, I don't mind the errors, I'm glad I've got an Ambassador-class starship (and from what I've read, it is perhaps quite literally all of them in one, with the Enterprise C name slapped on it). As for Deep Space Nine, I'm waiting for a 25% off sale for that one so I can save about £5 on it (it still looks really nice despite the pylon issue). Mind you, why would they angle the lower pylons the way they did so it would stand by itself? The schematics for DS9 show that they were flat on the bottom, wouldn't this have allowed the model to stand just fine?
As for my Defiant...any ideas? I have an old one with the Delta insignia error on that I could experiment with first, but I don't want to go ruining my new one.
I don't have the Enterprise-C or DS9, but I do have the Defiant. Apart from looking perhaps a bit more bright and colorful than a warship should, I really like it.
The pennant on the nacelles are facing the right way on mine, so that's not a problem. The nose section with the deflector isn't perfectly straight, but it's not really noticeable unless you look close. I like that they show it's not completely attached to the rest of the ship, but connected by a small piece, like on the show.
If yours is off enough that if bothers you (I have the same problem with the pod on the Nebula class), the only you can do, short of buying another, is to try and disconnect it and then reconnect it with some modeler's glue.
I don't recall the brand name (at work right now) but I got a really good kind that was really useful when a nacelle on my Equinox broke off.