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With 1.21 you're probably one of the lucky few that had no issues...
I have stuck with 1.21 for several months due to the complaints about 1.3x. And no ability to go backward if it is not stable (for me). I think I have A2 hardware.
1.21 has been very stable for me, but I do not use sharepoint. I'm not sure what 1.3x would improve for me, but at this point I am in the "don't fix it, if it isn't broken" mode.
Normally, I would be running the latest firmware.
@fd20092009 - The DIR-655 has been the best router I have ever owned. In general, the majority of people are happy with it (check Amazon and Newegg ratings). I have just stuck with firmware v1.21. I think there are several people who have upgraded to 1.3x w/o issue. The only thing holding me back is the inability to go backwards.