I have a Cobra, and it works very well.
The cobra is a air/nitrox/gauge computer with air integration. That means that in addition to calulating NDLs and ascent rates it also tracks your air usage and projects the amount of air time you have left.
It does do decompression calculations up to 99 munites of deco time. That is plenty for most folks if you want to go into simple (single gas) extended range diving. It does have a gauge mode for use with gas-switch deco and trimix diving as well. BTW, if you do miss a stop is locks you out, so be aware.
The SK7 is the compass that matches that computer. There are two versions. One that sits on top of the cobra and one that attaches to the back. They are known generally as the long package and the short package respectively.
There is also a HP quick-disconnect available for the Cobra. That is probably the QD you mentioned.
Personally I have the cobra with SK7 in the long package with the quick disconnect and like it a lot. I would also recommend getting a replacement guard to put in your kit. If the guard gets scratched it is easily replaced, but it really hurts the legibility of the computer until you do.
Any other questions?
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Jeremy