I use Threadmarks Pro on a large site with over 20 million posts. For over a year it's given me no problems. Users love it, because it allows people to keep track of the most meaningful posts in a thread even when it's filled with lots of chatter. Often used for threads of 10,000+ posts.
Upgrades to the core XF mean most functionality required is built in for me however this is the one essential add-on for our forum that is indispensable. I'm the user that's been submitting the bug reports for upgrading from XF1 v1.3.x (if it ain't broke don't fixit :) ) So as you can see the response and support to get this working has been brilliant. Thoroughly recommended.
Due to the unique nature of my forum, this has become a huge stable in the feature set I provide my users. I see this being useful for many other genres of forums as well as an excellent method of organizing threads together.