Summary: I have used 5300G for a few years now. The main problem is that the cameras periodically hang and the only way to fix the problem is to power cycle them. The motion detection is a bit hit and miss but once detected the e-mail notification etc. works well. I purchased the DCS6620G because of its optical zoom. The picture quality is exceptional and the zoom works well but is somewhat jerky.
Summary: I bought 3 of these and also bought about 14 of the 5300 model. These more expensive cameras with an optical zoom lens seemed to be good at first but they all eventually failed in the same way. The video has gone completely black on all three after about a year. The audio stil works and I suppose I could still use them as a 2-way comunication device, but that expensive color video with zoom is now useless. I tried running one again today just in case... but no luck.
Summary: This camera seems pretty good from the specs, but buried in the description is that you can view the images with Internet Explorer only! It turns out that it uses an ActiveX control for showing the video. Do these people think it's still 1998? There are people at my work using Macs and Linux machines, and there is no way for them to access the video feed. Haven't these people ever looked at YouTube?
Summary: I have a DCS-6620G and DCS-5300G. They both developed minor problems shortly after the 1 year warranty ran out. The DCS-6620G ethernet port failed and the DCS-5300G pan capability failed. I called DLink tech support and they told me that they will not repair these products or exchange them after the warranty, and that there are no authorized service centers.