(UPDATE(13.MAR.2013.):
There will be a H4 soon. I'm not sure when but I'm working on it. In the meantime I would be grateful for any and all info about your experience with it so far via PM either here or on the new Ultimaker forum. I hope that will make the next one even better and more user friendly. Thanks!)

After many trials and errors here is my new extruder for the Ultimaker.

It has some nice features namely:

-spring loaded bearing idler, based on Bertho's excellent design
-integrated bowden clip
-quick release system to pull out the entire gear/bearing/bolt assembly
-fliplock for very easy opening and closing without the need to release tension in order to open (something that seriously bugged me on the stock extruder, when I needed a lot of tension for a certain filament, the thing wouldn't open until I released the thumbscrew)

Take a look at the following videos to see how it looks:

http://youtu.be/17JNsZ7IKTU
http://youtu.be/Yb96mQMyrVk

Tested and working with the UM V2 bolt, Arcol.hu's hyena bolt and my KNRLR spiked bolts.

-UPDATE 05.OCT.2012-
A small change is necessary in the bearing axle part to make it more rugged, it's usable as it is but will deteriorate after a month or two of use. The best thing would be to use a 12 mm metal rod cut to 5mm thickness and drill a M3 hole in it. Or you can print spares and use that. I will try to design a new, tougher axle now and post results.

-UPDATE TWO 05.OCT.2012-
Solution found! Scratch the metal rod part. It's not exactly 12 mm and drilling a hole is a hassle. Get 3 M5 washers, they are round 11.8mm OD. Superglue them together. Take two normal m3 nuts and tape one or two loops of blue tape around them, cut off the excess width of the blue tape. Push that into the hole in the washers. And there you have it, a 5 minute all metal axle! :) Enjoy.

-UPDATE 07.OCT.2012.-
Added experimental kit for using the Ultimaker V3 bolt. Instructions will be updated soon. For some reason the UM mount was mirrored, so it's been updated with the right version.