They are relatively easy to install.
Having done a 97 SSEi install on my own car. Their are a couple of hardships to overcome.
On the front. Silly heatshield on right side (PS) of motor. A bolt on the front of the motor, you can sneak past the hose and belt..easy enough..then underneath there'* another bolt holding it to the dipstick. That'* the tough one. Not impossible..simply time consuming to get at it.
The back. Not bad to get at, only thing holding up back there were the exhaust pipe to manifold studs. A good drenching of PB blaster should take care of this problem.
Overall..quick and easy. I didn't remove the airbox to do the install. Simply started off soaking all bolts w/PB and starting on the bolts going into the heads (all came out nice and easy).
I picked up my manifolds from Hulkspeed and am extremely happy with the quality and price. OleBlueEyes has another source that is less costly, however I can not vouch for quality as I haven't seen them. (And their flow numbers seem a bit off for stock manifolds..but that'* another topic)
00 Regal GS: All kinds of cool stuff.