i wouldn't even consider buying an aftermarket deck, living in the city. Detachable face or not, that screams "steal me". theives know where there's an aftermarket deck there's usually aftermarket amps and speakers.
i'm probably gonna look into an FM modulator, one that doesn't transmit, but you plug the antenna lead into it, plug it into the antenna jack on the radio, and plug your portable into the RCA inputs. sound in these is typically better than the transmitters, you just have to make sure you ground it to the radio or you get engine noise. most people will find the fidelity in your FM tuner is pretty decent, not higher than the preamp section but much higher than the tape heads.
one other option for me is a kit to plug into the cd changer input for my honda head unit - you can get them on installer.com but they are like $100 - a bit high to still have stock sound...
what i'd really like to do is get a fatty head unit, mount it recessed 1", slice the faceplate off my honda radio and glue half a cd onto it. we used to do this when i was an installer (back then we used tapes, mostly)