Mentoring is working out GREAT for us.
We did the level 10 guild raid 2 nights ago and was able to mentor down our 2 groups to 25 so the mobs would be non-grey, we would get some drops from the boss mobs, and we actually faced a bit of danger in doing the raid. Took a bit of time to get spell bars updated (we tried at a couple of different levels until the grey was gone) but was well worth it.
Have also mentored some friends in the guild more than 10 levels lower than me. Was fun going back to varsoons at level 25 only to realize we couldn't even stay in the entrance area for long at that level (can you say group wipe?)
Forgot how dangerous it was when we first went there. Am looking to taking that group to the Giants in TS to help them get xp and maybe finish the giant language quest for me.
This is the BEST change that they've put into the game since it came out. I see using it quite often to group with friends and help others out. Also is good to open up a few quests that i missed out on.