Most street springs have progressive spring rates; coilover sleeves such as Ground Control use linear spring rates = worsened ride quality but better handling. However, the cheaper coilover sleeves/systems are not worth buying period.
It really depends on how much you want to spend. I have adjusted my ride height several times since I put on larger wheels (fender rubbing problem). My shocks are adjustable (rebound only) so I've tweaked with them during track days and whatnot. I'm definitely a proponent for some degree of adjustability, but if you're aiming to spend less than $300 a good set of springs will do you best.
Shingo