Originally Posted by
egear
The largest rear cassette you can use is a 34t. You can use a 36 but need to have a RD that floats. The 9 speed stuff is not compatible with the 10 speed stuff. Lets start with the crank. If its a 10 speed crank then you must use a 10 speed chain and cassette, 9 speed stuff will bind up on the chainrings. If its 9 speed then you are set. The 9 speed stuff is usually cheaper but some stuff like barends is beginning to get slightly difficult to find. If you have 9 speed then a 34t cassette should be functional but not with that Ultegra RD. Short cage is designed for road use and a 28 is about the best you can do with that setup. I would stay away from the M5xxx series as I have read alot of poor reviews about it's performance. Jensonusa.com has a handy search feature to help you narrow down your choices.
Parts of this post don't make sense. The largest cog on a Shimano cassette that is available is a 36. The XT and Deore and M5xx RDs can all handle that if they are long cage. The Ultegra is unlikely to handle anything above 30T because it is mid or short cage in length (if short cage, then even a 30T might be moot).
I use 9sp on 10sp chainrings all the time without issue. And here's the cruncher -- I use a 9sp cassette with 10sp STI shifters. The differences in width on the chain and the pull with road shifters isn't worth worrying about, in my opinion and experience.
As to the durability of the M5xx series, the reviews you've read are likely very minimal and from people who abuse their equipment than touring cyclists. I've not had any issues with Deore level RDs or cranks, nor LX nor XT. But if you ride your bike like you want to destroy it, you probably will have issues.