I haven't had any issues for my normal everyday use. Web browsing has been fine. I played over xbox live last night and it was fine. My one Netflix streaming test for 10 minutes to my Tivo worked better than it had been with my old router. I never run any sort of benchmarking, other than the usual Internet speed tests from the usual sites like speakeasy and speedtest.net. So far, it does seem that my upload is a few kbps slower than it was with my old DI-524. I was getting between 924-962 kbps across six tests I recorded going back to Feb '10. So far, with the ESR-9855G I've had between 887-903 kbps, for the past few days. Download has been a few kbps faster or basically the same. Of course, with cable modems, I can't say that if I hooked up my old router right now, I wouldn't see the same slower upload benchmark today. Stupidly, I forgot to do a test right before swapping out routers. In any case, my personal expectations have been met so far.
As for wi-fi, it is vastly superior to the poor wireless I had with my old DI-524.
Sometimes it would take up to a minute for my old Dell laptop to authenticate and my Nokia N810 wouldn't connect for up to 3 minutes after turning it on, unless I forced the issue. With the ESR-9855G, my laptop and Nokia are connecting within a second or two, which is clearly a huge improvement. Obviously, I lived with such nonsense for a long time, because I don't do anything like gaming or file copying across wireless in my house. For those who depend on wi-fi for gaming or video streaming, well, I really couldn't say how well it would work, but I haven't had any disconnections or anything when I have had my laptop connected for e-mail and web browsing or playing a video file stored on my old Buffalo (original 160GB) Linkstation. So, again, my personal expectations have been met so far and this is clearly an upgrade from my old router, but my assumption is that most people on this site have far greater expectations or needs than I do.
I did just do a file copy of a very large backup RAR file (a little less than 5GB) to my old Buffalo Linkstation from my PC and it took 15 minutes. I then copied it back to my PC and it took 11 minutes.
My next purchase is likely to be a Synology NAS which I expect to be a huge improvement over my old Linkstation.