I'm in the market, as it were, for a new bike. I've been riding my full suspension MTB to work (13 mi RT) since spring and I'm ready to save it for the dirt and get a 2nd bike.

I'm up around 230 and I have a decent downhill that I hit at over 40 on my low geared MTB, knobby tires at 45 psi. There is a couple of stop-lights on the hill that sometimes get me hitting the disc brakes hard - and they work great.

I live in So-Cal so rain is rare, but I'd like to be able to ride in it once in a while when I have to.

Regarding disc brakes, I'm picking up conflicting views. Any LBS tells me I need them or I don't need them, depending on the bike they are trying to sell me. It appears the Specialized Sirrus no longer has the disc brakes on the 2007. The Trek 7.5 FX has a version with them, but the higher end FX bikes (7.6/7.7 don't have them. I was told they are heavy and the hydraulic ones on my MTB are much better than the mechanical ones on these road/urban bikes.

At this point, I don't know if I really need them or not. I guess if I do get them, I can easily upgrade them to a better disc without getting new forks if they are "that bad". If I don't get them, I'm worried about being able to stop down that hill or in the rain. Also, some of the roads I get on are a bit rough, and I'm worried about keeping my wheels true (I ride pretty hard) and then the brakes will be less effective.

Are these mechanical brakes any better than standard brakes?

I'd like to get some opinions on this as I ponder my bike choice.

Disc brakes are massive overkill in almost all road situations. The only scenario I cank think of where they might be useful on a commuter bike would be riding through salty slush that would destroy rim braking surface, which you obviously don't have in California.

I would say generally one does not need disc brakes except for some hardcore mountain bike applications. People have been going 40 downhill in the rain for a long time, you know? Being better doesn't mean you need them. If you're worried about riding too rough for caliper brakes and road wheels then maybe you should look at a something more geared towared touring or cyclocross, with beefier wheels and cantilever brakes.

Good pads on caliper or canti brakes can make a huge difference in stopping feel and power. I'd say you don't need the disc brakes and associated complexity. Just swap out the stock pads w/ Kool Stop salmons and you'll be fine.

I'm switching to disc brakes before winter, but pretty much all my reasons wouldn't apply to you.
1-the rim brakes aren't so good in freezing rain and other icy conditions
2-during the mud season, the sand and crap that gets on the rims and brake pads really chews up the rims, and the chewed-up rims in turn chew up pads the rest of the year, so I go through pads fast, and my front rim is starting to worry me; I don't want a split rim at speed.

You don't NEED brakes either.
Personally discs modulate a lot better through a multitude of weather conditions. And good luck using your rim brakes in urban rain when its coated with sand or grease.

My discs are not 100% needed but they give me a huge margin of error and make life more comfortable. Not to mention they keep my rims much cleaner, esp when wet.

I have actually surprised myself with how fast my bike stops in the rain. I've found myself trying not to lock the rear wheel. Caliper brakes work just fine.

If you're locking the rear wheel, that means your brakes (or you) are increasing the pressure too suddenly, kinda like it's all on or all off.
My discs remain pretty linear no matter how fast I grab my brakes. I mean I can lock them but it's harder to lock my discs than my regular caliper brakes and I can better control how I decelerate with them.
Technically you can just get rid of your brakes and go fixed.

Disc brakes are awesome as far as braking power and smoothness are concerned, but they are expensive, heavy and a hassle to maintain. I would get regular old-fashioned rim brakes. But I also wouldn't go down that hill at 40 mph when it's raining, disc brakes or not (for one thing, surfaces are much more slippery in the rain, so I'd be worrying about losing control...). If you insist on those speeds in the rain, disc brakes might have a real safety advantage.

Disc's a hassle to maintain?

I used to have to install new pads once a month in the winter, less in summer.
I used to have to re-adjust brakes once a week in winter, less in summer.

I have barely looked at my brakes since converting to disc in close to a year except once every few weeks turning a large red dial one click. Thats as close to zero maintanence as i can get. Heck i can adjust them while waiting at a traffic light without getting off my bike.

Then again I commute year round in Vancouver, and live on the side of a mountain. Lots of hill, lots of rain. For me they are probably the single biggest noticable improvement alteration I did to my bike. If your in Arizona where its both flat and dry, I highly doubt you would notice any improvement.

For me, going downhill at 60 kph at 6:30 in the morning in the pouring rain in heavy traffic surrounded by lousy drivers, it was an easy decision that i have no regrets over. ymmv

Well, perhaps I'm just scared by their mechanical complexity. But still, isn't one supposed to replace the fluid in hydraulic-type brakes occasionally... Then, they may start rubbing... (I know, the rim ones do as well, but I guess I am more familiar with wheel truing than with disc truing - it's probably just a personal bias).

Yeah, I agree that for going downhill fast in wet weather, disc brakes are great. But it's probably safer to slow down altogether under those conditions...

Need, no. But I've owned bikes with and without discs and discs stop better, period. That said, I sold a bike with discs and now have a bike with cantis. Do I miss the discs, sure, but I still manage to stop with cantis taken off a late 80's mountain bike. I may change those out for some v-brakes at some point.

IMHO, disc desirablilty is, in addition to weather and hill grade, directly related to load weight. I had never had a problem stopping with my canti's...until my first heavier-than normal loaded trip. Shook me up a bit how much distance it took to stop and made me give a long pause to thinking about potential emergency maneauvers.
I have no desire to rear-end some car that hits the brakes suddenly just after passing me on a narrow road, bridge, or in a tunnel.

I have mechanical disc brakes and this is literally what I do:

Turn both adjustment knobs on the calipers all the way out.
Rip old pads out, install new pads.
Readjust knobs until the travel is right.

That's it. And when the levers get softer, I just give the knob a few clicks and I'm done. Pretty easy.

I got mechanical disc brakes and if I wasn't such an rain rider I would say no, but for rain they are awesome. No worrying about pad condition/dampness, they just always stop the same. On this particular bike the weight issue doesn't effect me as things like the bike lock, lights, etc would go out the door first.

It doesnt rain all that much in the SF Bay, but it rains even less in LA.

I love having discs on my all weather commuting bike as they work great in the rain. The mechanical brakes are excelent and need little maintaining.
If I am going to ride in the rain, I want discs and fenders.