I have a DIR-655. It's fast, and the QoS and WISH features are great. But, I have had issues with numerous firmware releases. My router shipped with firmware 1.04. I tried to upgrade to both 1.05 and 1.10 when they were released, but the router was unable to get an IP address from my cable modem with either of those two releases, so I had to downgrade back to 1.04 each time. This was fixed in 1.11, which I was finally able to upgrade to and which worked great. However, firmware 1.20 and 1.21 introduced some new features that I'd like - other uses for the USB port, and wireless guest zone - so I tried to upgrade to those. With either firmware, LAN printing to a shared printer on my Vista x64 PC is unusably slow. File sharing still works fine, though, and downgrading the firmware back to 1.11 fixes the problem.
So, since I got my DIR-655, there have only been two out of 6 firmware releases that haven't contained showstopping bugs that have made the router entirely unusable for me. As it is, I am still without the new features that I'd like to have, because the LAN printing issue was introduced in 1.20 and not fixed in 1.21 (even the latest build 8 without SecureSpot).
I'm still happy with the router in general, because with firmware 1.11 it does everything it was advertised to do and does it well. But it's a pain to install a new firmware, restore all of my settings manually (I don't bother saving the settings to a file and restoring them from there, because that hardly ever seems to work properly), find out that it doesn't work right, and spend even more time reverting the firmware and the settings to the previous ones. So it would be nice if the next version of the firmware could fix the LAN printing problem without making the router catch on fire or something.