Ronnie: Be afraid. Be very afraid.
Seth Brundle: My teeth have begun to fall out. The medicine cabinet is now the Brundle Museum of Natural History. You wanna see what else is in it?
Seth Brundle: I think you're making a mistake. I think you really want to talk to me.
Ronnie: Sorry, I have three other interviews to do before this party's over.
Seth Brundle: Yeah, but they're not working on something that'll change the world as we know it.
Ronnie: They say they are.
Seth Brundle: Yeah, but they're lying. I'm not.
Seth Brundle: I was not pure. The teleporter insists on inner pure. I was not pure.
Ronnie: I don't know what you mean.
Seth Brundle: A fly... got into the... transmitter pod with me that first time, when I was alone. The computer... got confused - there weren't supposed to be two separate genetic patterns - and it decided to... uhh... splice us together. It mated us, me and the fly. We hadn't even been properly introduced.
Tawny: Are you a body builder, or something?
Seth Brundle: Yeah, I build bodies. I take them apart, and put them back together again.
Ronnie: Don't you get it? I am finally onto something that's big. Huge.
Stathis Borans: What? His cock?
Ronnie: Crude Stathis. Even for you.
Seth Brundle: There was an old lady who swallowed a fly, perhaps she'll die.
Ronnie: If you SAW him, Stathis, if you saw how scared and angry and desperate he is.
Stathis Borans: I'm sure Typhoid Mary was a very nice person too when you saw her socially.
Answer: Brundle's just had all of his atoms separated and then joined back together with a massive influx of electrical energy; certainly that would generate some sort of sensation in cells that have never experienced it before. No reason to believe it couldn't manifest as euphoria and strength. As far as the transformation: Brundle's cells have been put back together with fly DNA in them, but with very little actual fly material, initially. It's reasonable to expect that as his cells die off and regenerate in the usual ways, they are replaced not by human ones, but by human/fly hybrids. As this happens, he becomes gradually more fly-like.
Rooster of Doom