I tried out the xCoupler while we were searching around for alternatives (hint) to RSSql/FT Trans. manager, and it is indeed pretty awesome. Unfortunately it is also pretty expensive. I was quoted around 10k for one in September '07. It will definitely be worth it if you need to do fast logging, since it plugs directly into the backplane, but for what I do it was overkill.
To me, RSSql is a pain to configure, even if you know it well. I also have issues where after starting it, it does not recognize triggers for a couple minutes. I also had HUGE problems on a larger scale project (around 12,000) tags using KepServer. It would constanly fail and stop responding to the PLC, and it was not KepServer's fault. Rockwell support couldn't fix it. If you do go RSSql, I would recommend using RSLinx for your OPC server, since Rockwell should work best with Rockwell. RSSql can also only talk to 1 database at a time, which is a problem for me on a different project. Bi-directional communication apart from transaction handshakes is also pretty much impossible without stored procedures.
You might also want to look into FactorySQL from IA -
http://inductiveautomation.com. I like their product the best out of the transaction managers I've tried, and hey, it happens to be the cheapest as well. And none of this tag count licencing. It can also talk to as many databases as you want. Bi-directional communication is a breeze. Still have to buy an external OPC server, though.
Hope that helps!
-pandersen