Lot of good 'what to buy' guides out there based on xp earned, gear obtained, and quality of the content. I've not bought anything for DDO yet either, but some of my friends are pretty far along and I figured if I liked it I'd save a bit more than paying a monthly fee.
You don't saved much, but being able to access it at the speed and time you want to access it is a nice option.
Breaking all of the pots has killed me tho'. Also the actionRPG feel is nice, but the same animations got stale for me pretty quickly.