I had also been looking to do this for a while, so I thought I'd chime in here with some feedback since I recently took the plunge. I wanted to create a sort of "docking station" for when I'm working at my desk.
I'm very happy with this configuration and it works well for me but YMMV of course. I don't use my computer for gaming or graphics work aside from photos. I mainly use the secondary monitor as extra space while working with spreadsheets, reference material and documents and I desperately needed more screen real estate so that I didn't have to switch back and forth between windows while working. Personally I feel it is much more efficient to work this way and it feels like a relief from just using the standard laptop screen.
I did look for, and notice some "lag". But it's almost imperceptible and doesn't affect performance for the tasks I mentioned. If I wasn't looking for it I probably wouldn't have noticed and it doesn't really bother me. I am able to play a full 1080P movie in quicktime on either monitor and can move the window while it's playing to either screen, and even leave it in between screens so that it straddles both. It continues to pay fine with no hiccups. I also played two 1080P mp4 movies simultaneously, one on each monitor, and the result was fine. There are some issues with flash content displayed in a browser. Some flash advertisements and things such as the player scrub controls on youtube suffer from a very noticeable flicker. On my system I am able to play a 720P youtube video in a window or full screen on the secondary monitor and the video itself looks fine as long as it has buffered enough (even with a 1080P movie playing on the primary monitor). However, these known issues with flash and browser content on the displaylink device could definitely be annoying for some people. The simple solution is to use the primary monitor connected to the mac for content such as this if it bothers you. For me this isn't a big issue since I use the secondary displaylink screen for work and I understand that some multimedia or 3d applications may perform better on the primary screen.
Here's my configuration:
2012 13" Macbook Air, i5 with 8gb RAM running mountain lion 10.8.4
(2) Asus VS228H-P 22" Monitors (both using the HDMI input)
Planar dual monitor stand
HDE Mini display port to HDMI cable - connecting the MBA to the primary monitor
Regular HDMI cable - connecting the plugable device to the secondary monitor
Plugable USB 3.0 to HDMI adapter (connected to the MBA via included USB cable) Drivers were installed via the website before I plugged the device in.
Twelve South Bookarc stand
Apple wireless keyboard & mouse