I just made the plunge in the past week to a 29er. I was commuting a few days a week on my road bike (aluminum frame, carbon fork, 700 x 28 C tires) and was tired of getting abused by the road conditions and suffering a high number of flats. My commute is a little over 8 miles over poor road conditions and substantial elevation changes (300+ ft elevation, 10% grades). I have a 26" MTB, but using it for the commute is out of the question simply because it does not climb well at all. I picked up a 29er with a front shock (and lockout) to tame the ride and a beefier frame to handle some light XC duties my MTB would be used for.

After a week of riding, I don't regret it a bit. The 29er does have a bit more weight than my road bike and, combined with the wider and more treaded tires, is about 10 - 15% more difficult to climb. The 29er also loses speed faster due to increased rolling friction. Surprisingly, though, the 29er's gearing really does quite well at handling the steep climbs and the long, fast downhills. Again, I do have to gear down a bit further on the 29er to keep the pace but it really isn't as much as I anticipated. Given the relatively modest shortcomings, the 29er more than makes up for the difference in ride comfort. The ride is a ton more comfortable and I find the cockpit ergonomics well suited to the fast open road and the tight downtown traffic. It's a great all-rounder for 90% of the riding I do, and it offers me the versatility to tailor it a bit to a specific use without changing the bike.

In the end, it is a trade-off but you have to look at what is most important to you. If you're willing to sacrifice a bit of weight and low-speed agility for a more comfortable ride and better high-speed stability, the 29er may be what you're looking for.