I feel I need to clarify some things:
Q: Are mechanical discs worth the $$$$?
A: Only if you get Avids, or Hayes, (Verdict still out on the Deore ones, but have been getting a so-so rating)
Q: Are discs better than Vee brakes?
A: Yes, if you're into Downhill, or extreme Freeriding where there are long downhill section, and/or extremely muddy conditions.
Q: Are discs worth the extra $$$ over vee-brakes? (Similar to ? above, but with a different point-of-view.)
A: Not for 90% of us. But they are cool! Nothing says you can't or shouldn't have them if you've got money to spend.
Q: Does everyone need discs?
A: No way, vee brakes have great stopping power and are efficient and relatively inexpensive. Lighter weight too!
Q: Are hydraulics better than mechanicals?
A: In most cases, yes!(if you buy quality of both) A side-by-side comparison will show that hydros have better feel, more power and better modulation.
It all boils down to A) what type of riding do you do? and B) how much are you willing to spend?
L8R
BTW, I've got 2 mtn bikes one with vee-brakes (Avid's) and one with discs (Hydros). BOTH STOP when I need them to!
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"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming, "WOW, What a Ride!" - unknown
"Your Bike Sucks" - Sky Yaeger