Just got my new dock a few days ago and decided to post some pictures and write a quick review since when I was looking to purchase it I couldn't find much info online.
First some details on the dock. I have a W520 which comes with a 170W power brick (literally the size of a brick) so I had to order the Mini Dock Plus Series 3 (170W) which comes with the same power adapter. I'm not sure if there is any difference between the 170W and the 90 or 135 W other than the supplied power adapter as the model number on the bottom of the unit (4338) is shared among all of them but the Part numbers when ordering are slightly different (the dashes aren't typically needed):
ThinkPad Mini Dock Plus Series 3 90W - 4338-10U
ThinkPad Mini Dock Plus Series 3 135W - 4338-20U
ThinkPad Mini Dock Plus Series 3 170W - 4338-30U
Quick side note: I have heard the W520 will run off of a 135W power adapter which would be nice to use for travelling but didn't want to risk ordering the 135W dock and having it not work.
The dock I ordered was a 433830U and it has the following part numbers on the bottom: P/N 75Y5730 and FRU P/N 75Y5731 Interestingly the specs say 20V 4.5A which corresponds to 90W. From talking with others it appears that internally all of the Mini Dock Plus Series 3 are identical but they come with different power adapters and may or may not have a keyed power socket.
The keyed plug is to prevent the 170W adapter from being plugged into an older ThinkPad, you can still plug an older ThinkPad's power adapter into this dock. The power adapter that came with my 170W dock was the exact same as the one that came with my laptop, P/N 45N0113 and FRU P/N 45N0114, 20V 8.5A (170W).
Here is a closeup of the keyed socket and 170W power adapter plug:
The dock itself seems quite well manufactured but it better be given the price (269.99 CAD). It consists of a narrow wedge with a large cube on the back left corner. The cube has a locating device to make it easy to insert a laptop into the dock as well as the eject button, a power button, and the key lock.
The backside of the wedge has an absolutely enormous amount of ports which I will list in order starting from the back left side of the unit:
This is a pretty awesome selection and while I personally would've liked to see some HDMI ports I can totally understand why Lenovo chose to use the royalty free DisplayPort instead. Since DisplayPort to HDMI cables are available it isn't a problem just slightly inconvenient given that monitor manufacturers seem to all love HDMI.
My one MAJOR complaint is the location of the audio connectors. They are mounted on the right side of the dock and are completely inaccessible when a computer is docked. I guess they are intended for desktop speakers and a microphone but I'd like to be able to use them to plug in my gaming headphones that have a speaker and a microphone jack (the W520 does not have separate speaker and microphone jacks requiring a two to one adapter to use dual jack headphones).
The other big feature of the dock is the ability to use multiple displays. Lenovo claims three external displays plus the laptops display itself can be driven if using the dock and an Optimus enabled laptop. I have yet to try four displays but found it worked well for a single external monitor.
Overall a very nicely engineered piece of hardware. I'm quite satisfied and probably will purchase another dock with my next laptop. If you are constantly unpacking your laptop its nice to have a dock in at least one location.