Just sayin - I thought I'd use this tool like once or twice only when I got it, figured my RD would never ever again get out of whack, but I've been using it every year despite taking care not to lay my bike down on the RD, etc. Was recabling my RD today due to errant shifting in the back, and lo and behold, the DAG tool back to the rescue - fixed a surprisingly large hangar adjustment, and suddenly everything is back to 100% perfect in rear shifting, did not even require any indexing or any tricks for cable tension.
If you're interested in having perfect indexed shifting and doing the work yourself, I'd say this is a 100% must-have. It seems like an expensive tool at first ($80-$90 for the excellent Park tool one) but honestly, it'll save you way more than that in time. I can't tell you how many hours I've spent messing around with the RD cable tension, barrel adjusters, etc, when the real culprit was the hangar. In fact, I think every time I've gotten the hangar aligment correct, I didn't have to adjust anything else on the RD at all. (But yes, I already did the limit screws, and every time I recable, I make sure that I can't shift the RD into the spokes - have seen that before and ends really badly!)
If you're interested in having perfect indexed shifting and doing the work yourself, I'd say this is a 100% must-have. It seems like an expensive tool at first ($80-$90 for the excellent Park tool one) but honestly, it'll save you way more than that in time. I can't tell you how many hours I've spent messing around with the RD cable tension, barrel adjusters, etc, when the real culprit was the hangar. In fact, I think every time I've gotten the hangar aligment correct, I didn't have to adjust anything else on the RD at all. (But yes, I already did the limit screws, and every time I recable, I make sure that I can't shift the RD into the spokes - have seen that before and ends really badly!)