Anytime.
When I bought mine, I looked for an affordable one that specifically works with DD-WRT. I wanted DD-WRT on it because it's so much better stability- and feature-wise than the default firmware on many routers, including more expensive ones like Linksys or Netgear.
Unfortunately, in Europe only D1 and D3 revisions are sold as retail DIR. D2 and D4 are made specifically for ISPs like Virgin Media, who then give them to their customers. I can live with my D1 though.
My point is that with these home routers it's not the hardware that's faulty or problematic most of the time, but the factory firmware. With this in mind you might want to keep your DIR-615 D1 with DD-WRT, or get another model officially supported by DD-WRT, as regardless of what brand you buy, the firmware might give you headaches.