Hi there,
I've been using DL 1.6.6 with Leopard, with all the available updates, for more than a month on a MacBook Pro (Santa Rosa) without any noticeable issues.
The library becomes a bit sluggish as it grows but this is a DL 1.0 problem that will, hopefully, be resolved in the new version.
As far as I know, if you purchase the DL now, you will be eligible for a free upgrade to 2.0 when it finally ships. This is the reason I bought mine last month.
Hope this helps. (Judging by the previews, can't wait for the 2.0 to ship, it will be gorgeous)