I have used a VNC server (tight-vnc) as opposed to a KVM and I have been happy with the performance. THe delay is almost negligible if you are not playing full motion video or gaming. For normal office apps and web browsing even it is very quick,robust, and reliable on a LAN. Even playing a small movie clip works fine, just not full motion video.
A serial console is not really neccessary unless you are running a keyboard/mouseless networked server that you either need to go to user level 1 with and/or you want that to be your only point of administration (I think, but I'm going off of memory here).