I went through several tools in the last 3 years and can offer my short comparison.
To begin with, I had the complete SD tools and i must say i would have delete this experience from my past
total waste of time and money.
I have been buying and using DFL tools since they started and i love these guys, the products are still a bit raw and luck of proper stability to my opinion but they are working very hard to improve it and listening to customers in regards to bug fixes and addition of new functions. they are improving all the time and the key here is that they are listening to their customers and sharing the info inside their forum and they will always help you out when you need their support, i would highly recommend them, there is nothing similar to SD here like most people are afraid from past experience.
I have also DDIv3 for 3 years and this is my favorite imager from all of them.
And 7 months ago i bought the PC3K Express and PC3K Flash.
As a product i can say that there is nothing like the PC3K + DE as it is a complete suite that offers support for all type of drives and the both the FW utilities and DE have many many functions that i am still learning.
The PC3K + DE is the most stable product and sits on one PCiE without all the hassle of many external boxes.
Problem with ACE is the delay in the support and the cost of product and support.
In regards to DFL they still don't offer a complete suite for all brands but already covers WD and SEAGATE and the DE and DDP and now the URE which i still don't have and wait for some feedback.
I must say that in many cases i prefer to work with the DFL over the PC3K in FW mainly with WDII.
From my experience with the DFL products which i own:
1. WDII - to my opinion their best tool and very powerful in general.
2. STII - bought it about a month ago but didn't try it yet - looks ok but still raw.
3. DE - i had it for 6 months and i replaced it 6 months ago - to my opinion was very raw and unstable at the time so didn't use it much.
4. DDP - replaced my DE with it - it use to be unstable but since i bought it they released 4 versions which made it a good tool with many functions and much more stable - in general it is a good tool but to my opinion imaging ability is still not as good as DDI and PC3K DE. but DDP has many functions that the DDI don't and good user interface. in general it is a decent imager and functions around are not a must by nice to have.
5. URE - usb imaging new tool - don't have it yet but wait to here some feedback.
6. Support and Human interaction - brilliant - the best support on the industry no doubt.
i had the products since July 2012 and i was always impressed with the quick assistance and responsiveness. Especially from Mr. Stanley Morgan, he is a sweet helpful guy that always try to help.
Although in the last 2 months they grew very fast with many new users and i felt that support quality is decreasing slightly so i guess their real challenge is to stay with the same level of support with many users and not as a small company, if the'll manage to do so they'll have a big edge over the competition.
Hope that helps