RDM is a very special creature indeed.
I played BLM & WAR/NIN on Retail mostly endgame.
I wanted the solo abilities of RDM when i rolled on Nas due to low pop.
I can say that a RDM can solo 1-40 with not not much difficulty and at a pretty decent speed.
I tanked in Dunes, Qufim, Bibki as /war.
And DD'd as /nin
Ive DD'd (with good gear) in Garlige Citadel.
You can solo some sky pop NMs, Genbu, Roc, Bloodtear/Steelfleece and many other difficult mobs DDs could not.
You can tank and kite as RDM/PLD or as RDM/NIN with the right gear.
RDM may be the most complicated class to play because it can play many roles, though with groups you will be limited to mostly enfeebles and heals.
I find the most fun with RDM in low man and solo situations, I don't care for it much as part of a large group.
Hope this helps