Mine was on about 108k I guess, I did them back in Feb, and it's on 110k now. Symptom was a noise from the rear when going on/off the gas at low speeds. It would vary between a squeak and a clunk depending how hard you mashed the pedal. It sometimes sounded like the spare wheel was moving around, a rubber-on-metal noise, if you can imagine it. If you jack the car up and have a look at the bushes it should be obvious if they're shot, you'll be able to see where they've been moving about. THIS is what mine looked like. Not good !

I hired the tool, it cost me £40. They're about £200 to buy new. If you've got a buddy with an X5 that needs to do the same job it would make sense to buy the tool between you and sell it on ebay afterwards, splitting the cost. The bushes I got were Febi-Bilstein. They appeared to be exactly the same as OEM. Maybe they won't last as long but if they only do 30k miles instead of 90k+ that's a lot longer than I'm planning on keeping the car ! Definitely change all four. Pointless only doing two, IMO.

If you're going to do it, there's a couple of good write-ups on here somewhere. Not a hard job but you will need a gas torch to heat the exhaust hanger and bend it out of the way. Otherwise you need to remove the exhaust and that's a pig of a job.