Springs aren't a terrible ride and are relatively inexpensive at around $300. A coil-over step replaces springs and shocks and is apparently a much better ride, but usually at double to quadruple the price. I have H&R springs and couldn't justify the expense of coil-overs. I think it rides fine, but that's entirely subjective.
Search my threads/posts and you'll get all the info you need regarding lowering. I went with H&R's, along with MM spacers, and also home made "spacers" (which I dubbed the "ed-mod" because Edmund from here came up with the idea) for the rear because the springs alone lowered the rear too much with my larger wheels & tires.
Regarding the MM spacers, I wound up removing the stand-offs (the tubular spacers that went around the mounting bolts), so now the top bracket rests directly on the frame of the car. I did this because otherwise my rear shocks wouldn't auto-level at all (the spacers "tricked" the shocks too much). With the way I have it now (no stand-offs) if I have people in the back seat or an otherwise heavy load (a full tank of gas with couple large coolers loaded with beer and ice in the trunk, for example), the shocks still auto-level to nearly exactly what the load-free ride height is at. Between the new wheels & tires and lowering the car it took a LOT of tinkering and trial and error, but it's all perfect now... thanks to this (and the other) forum.
__________________ '06 CTS-V (Black on grey)~K&N FIPK II~muff & res delete~Magnaflow Tru-X~CS motor & trans mounts~PISNUOFF shifter~H&R sport springs (spaced up 3/8ths in the rear)~MM spacers~Z06 front endlinks~Hyper Black Linea Corse Venetos w/ 245/40-19 & 275/35-19~MAP trailing arms~eBay clear fogs~various LED conversions~Brake Motive D/S rotors~Gripforce LS7 clutch~