I agree with those that have said that when you start to realize the limitations of your current bike.

I just got a road bike and I love it. My rationale for getting it was that I wasn't able to go much further/faster on my MTB. A 60 mile ride on a MTB takes a lot out of you, especially if you are trying to, say, ride with other people!

I commute a little over 20 miles RT and do that on my MTB as opposed to the road bike. I've commuted a few times on the road bike but find that it is hard to carry all of my stuff (laptop, clothes, food) on the road bike. The aggressive position makes the weight difficult to carry, whereas on my MTB I am more upright and have a good rack bag/panniers. I chose the MTB over a hybrid/touring bike for commuting because one of my favorite routes home is a good singletrack but I think if commuting is one of your goals a road bike might not be the best option for that if you carry a lot.

I'd look for a good used bike or last year's model new (test ride a *lot*, preferably for longer distances if you can.) Know what you are looking for and make sure that you get a good deal. My road bike was last year's model and I got $600 off because of that.

I like the model for upgrading that Jiffers mentions but have heard that you get a much better deal on higher-end components when purchasing them on a bike as opposed to individually since the bike manufacturers buy them in high volumes. That would be easy enough to prove or disprove with some searching online.

I wouldn't say that climbing is better on a road bike, it really depends. On my commute I climb a couple of solid 8-10% grades. I find the climb a lot more difficult on the road bike because I am carrying a lot of weight and I don't have the easy gearing as on the mountain bike. On the mountain bike the hills are pretty painless but it definitely takes a lot less time on the road bike!

My advice would be to take your time, do plenty of reading and research online and then test ride bikes to make sure that you get one that fits you well. I am 5'10" and was told that 5'8" is about the cutoff for women-specific bikes. I always fit the 'standard' frame size better.

Anne