I used DxO when it was version 4. It had been out a while, and the lens distortion corrections really impressed me. It was the first RAW processor I used. In general the consensus regarding DxO was that the usability of the software was a bit kludgy, but if you put effort into it you could obtain superior results to what ACR could do at the time.
Then they released v5, to which I received a free upgrade (because I purchased v4 shortly before v5 was released). It was a steaming pile of junk. The software was released essentially unfinished, with many bugs/crashes and performance issues, missing core features (like photoshop integration), etc. IIRC it took them several months before they released a mac compatible version of v5. You can do a search now on various photo forums and you will see that there was not a lot of good comments to be had regarding the release of DxO v5.
If you read their forums on the release of v5, it was awful. Every post was some problem or another, and many were deleted by mods. It took them months and months just to acknowledge these issues let alone begin to address them. Shortly after using v5, I changed to using Lightroom (v1). Lightroom ran much better on my powerbook g4, and had a superior workflow for my needs. It is more of an integrated solution with everything from import to storage to output. DxO is more of a batch converter tool, meant to work into an existing workflow already.
I heard that eventually DxO v5 became decent (more than a year after its release), but at the same time Lightroom 2 came out and rapidly improved/matured the quality of the ACR engine. I don't know what response DxO v6 is getting, but again with the release of LR3 and AP3, the competition has kept up with or surpassed the advantages that DxO used to have IMHO. The only thing that DxO had going for it after a while was the geometry correction, and now that it has been introduced to LR3, there is not a single feature that DxO has that I can't get in LR3.
To summarize, after seeing the poor quality of the software and how badly they handled the release of DxO v5, the time it took them to acknowledge and fix the problems in the software, and their general lack of customer support and dedication to customer satisfaction, I did not find them to be a company worth supporting any further. All I can say is that I was glad I got my v5 upgrade for free and that I have not missed DxO since I switched to LR.