Well, they matter to the point of you usually NEED them to boost the mic level signal!
After that, they make a FAR less difference than:
1 - How well the person plays their instrument, and how good that instrument sounds!
2 - How the instrument was mic'ed.
3 - What the instrument was mic'ed with.
After that, it is just electronics, and the electronics tend to NOT difference anywhere as much as the big three above.
Too often, the "quality" of cheaper electronics is blamed for bad sound, when the reality is, the one of the above is the main cuprit. The reason I am so adamant about most guys around here worrying about preamps is because most here should really be concentrating on learning how to compensate in the MAJOR ways sound can be altered.
I have ran a SM 57 thru most every kind of popular preamp out there! From the cheap to insanely expensive. I have done my own preamp shoot-offs.
Did you know that when I mic up a electric guitar, and run it thru
a Focusrite Red 8, a Drawmer 1960, and a ART Pro, 10 out of 10 times the client prefers the sound of the ART? So do I!

Certainly NOT because the client likes it, but rather, because it just has a sound that works well! I have live recordings galore using cheap ol' Mackie pre's. Nothing wrong with the sound at all! Maybe not quite the sound I would always want, but serviceable, and again, I don't let the pre stop from from achieving a good recording.
The moral? Simply, yes, preamps make a difference, but not always a good one.

And, it is QUITE possible to do excellent sounding recordings using more "budget" preamps. If you have great source sounds, have spent some time with mic placement, and used the right mic for the purpose, the difference in pre's will be slight.
It would be nice if there were more questions around here about how to improve source sounds. Different ways to approach micing something. Specific mics for specific tasks WITH audio examples.
On this glorious Christmas day, I hope to bring HOPE to the masses that you don't have to try to buy your way into great sound. You need only to try, try, try again with what you got.
http://www.betteroffdad.com . Go listen to these guys mp3's. All the acoustic guitars and vocals are recorded with a AKG C 3000 via a DMP preamp. Most of the other tracks are contributed from friends (via another BBS) from around the world using mostly low "quality" preamps. This is some excellent sounding productions for the most part. Indeed, they can be improved, but, not by staggering amounts.
It is FAR more about learning how to use your tools than what tools you are using. Anybody that uses the excuse that their preamp is making or breaking their productions is the one in denial!
