I installed DD-WRT v24-sp2 (04/23/10) mega (SVN revision 14311) on the 3500L (the dd-wrt.v24-14311_NEWD-2_K2.6_big.bin file, to be precise), starting from factory 126.96.36.199_30.0.98NA firmware. Actually, I first installed the recommended dd-wrt.v24-14289_NEWD-2_K2.6_mini-WNR3500L.chk file to make the initial jump from factory firmware to DD-WRT, then the release mentioned above.
After the firmware upgrade, I did a recommended reset to factory defaults. The only changes made from defaults were to put the LAN client in DMZ, disable QoS and set the wireless channel to 1.
Figure 2: WNR3500L routing throughput - DD-WRT
For comparison, Figure 3 shows the same tests run with NETGEAR firmware.
Figure 3: WNR3500L routing throughput - NETGEAR firmware
Night and day, eh? I was quite surprised to see unidirectional routing throughput drop by up to 70% and routing stability significantly degrade. But I also noticed a high imbalance in WAN to LAN and LAN to WAN speed (29 Mbps LAN to WAN vs. 82 Mbps WAN to LAN) with data flowing in both directions simultaneously. I tried disabing SPI and also the WAN Traffic Counter (ttraff Daemon), but neither improved routing throughput or stability.
Some people like to load up DD-WRT because it allows them to increase the number of simultaneous sessions that the router will allow, which is important for online gaming and P2P file sharing. My tests found that the factory firmware allowed only 2045 concurrent sessions, which indicates a 2K (2048) session limit.
The DD-WRT GUI defaults to setting the IP Filter Settings (Administration > Management) to 4096 and that's what my test confirmed, topping out at 4081 sessions.
Figure 4: DD-WRT IP Filter settings
But the new trick that the 3500L picked up with DD-WRT was that it locked up after each of my maximum simultaneous session test runs, requiring a power cycle to bring it back. You can download a zip of the programs and instructions from here for the maximum simultaneous connections / sessions test, if you'd like to run your own tests.
Since the 3500L didn't handle 4K connections very well, I didn't try using the command line to increase the sessions even further. I also didn't try the recommendation higher up on that page to free up RAM by disabling the Web GUI and using the command line only to manage the router.
I've asked NETGEAR for confirmation and / or comment about my findings and will post back here when I hear back.