Staying in Koblenz vs. Cologne
 

My friend and I are arriving into Frankfurt on May 2, 2006. We had considered going straight to Cologne from there, but it appears to me that if we want to see the Castles in the Middle Rhine, that Koblenz might be the better bet. We are eventually going to Luxembourg and then the Amsterdam. Is there anything we do on a day trip to Cologne? or would staying there be better?

Koblenz is absolutely beautiful, and I would definitely stay there. As for Koln, the most important thing there is the cathedral, which could be done on a day trip. You will love Koblenz.
Cheers!

I'd go with Koblenz too if headed to Luxembourg next. From there you can train or boat south from Koblenz to see Marksburg Castle in Braubach (very good tour of intact castle.) On your daytrip to Cologne, plan to stop in Remagen (very good WW II museum housed in the supports of the old bridge there that saw many a battle) and/or the quaint village of Linz (on the east bank.)

Burg Eltz, Cochem, and Trier are worthwhile stops on the way to Luxembourg - more worthwhile than Luxembourg itself, I'd say. See them all, then you be the judge.

Koblenz!
And that castle that sits up on the cliffs actually offers cheap accomadations (hostel)or at least it used to. Regards!