I just got my new bike today and I swapped out the 11-30 it came with for an 11-34 without any hesitation. The 11-34 setup was one of the things I liked best on my old bike. In my case I didn't need to change the chain or derailler, I just needed the new cassette itself. It cost about $100. I have a di2 medium cage derailler on my bike. I find that any time I hit grades of 10% or more, I really appreciate having the lower gearing. I can put out a lot more power if I'm able to keep my cadence up in the 70-80 rpm range, rather than trying to grind it at a really low cadence.