I just picked up an AFDII for 1/4 that price...
Find some way of testing both before committing.
John was able to get the AFD II for such a good deal because I sold it to him after upgrading to a DL28 system.
I also agree with John when it comes to testing both. No one can tell which works for you best except you.
Regarding the Dl28; I did not pay the retail price (long story, but the gist of which is that I upgraded to this after having problems with my zd back). Though it's been debated, I think even at retail cost of $14999 plus the free lens, it is a solid deal if that is the direction you want to go. Consider that renting a back can cost upwards of $200-$400/day. Depending on your use and how you pass on your costs to customers, renting might be more cost effective if they pay direct costs, however if you can charge more for the "higher quality" imagery of using a MF digital back, passing on the costs through your fees, a DL28 system might pay for itself within a year or so.
As far as reviews, there are not many out there, mostly because it is not a "system" like other systems such as the AFi. Look up reviews of the digital back and the camera body separately. There is some info out there on the Aptus II backs, though much of it is on the Aptus II 7 or 10.
I am planning to post some files soon, but the holiday rush has caused me to slow down a little on my delivery of these files. My hunch however is that you are not going to get a definitive answer of whether or purchase the DL28 system or the 5DII. They are very different tools. I plan to use the DL28 for landscapes and portraits mostly for the detail, DR, and color rendition, but only in ISOs up to 400. Other work will be the D300 mostly (up through ISO 1600). I want to push the DL28 but there will always be circumstances where the D300 will be the better tool, despite lower mp, lower detail, etc. The AFD III is a decided step up above the AFD and the AFD II (though the jump is much smaller between the AFD II and AFD III). The auto focus is much faster, there are three focus points, the ergonomics are better, and the shutter has less lag while seeming to be softer producing less camera shake. However, it's still a MF camera, so it cannot compete with the focusing system of nearly all DSLRs (not just the high-end ones such as the 5DII or D700/D3, etc. I'm willing to say bet that the focus system of the AFD III is on par with something like the D60).
I don't feel comfortable giving you much advice, so consider the following with very large grains of salt: If you are just looking to get into MF (to experiment, etc) I might recommend renting or buying into a used system. Mamiya is great b/c the glass is quite solid but much less expensive than other brands. Maybe even an AFD or AFD II and a used back such as an Aptus 22, Phase One P20/P25, or even the old Kodak backs. If you are looking to shift into MF and want a higher level of support from a vendor, etc, then I think the DL28 system is great.
In the first case, you might be able to upgrade your DSLR to the 5DII while still getting to play with MF digital. With the second option, it might be a series of prioritizing where one system is purchased while the other is put off for a year or so (with innovation, of course, continuing to climb the ladder).
Feel free to PM me for more details on the DL28 if you're curious.