I have your Housing Sentry and it's the cat's ass. Acutally on 2 of my 5D MII Aquatica Housings. It saved my butt on a predive check so it paid for itself. As usual I was in a hurry to go after trying to fix my strobes (which weren't firing) and the boat was about to leave. Classic set up for disaster.
I do have the added insurance policy to cover both the camera and housing should it get flooded, lost or stolen. I do like the idea of a keeping the moisture alarm in the event the pressure switch fails. With the moisture sensor still working along with the vacuum still in play during a dive is good backup. In addition, God only knows what the baggage handlers do to your housing even when packed in a Pelican case. I have a few scars on the case already that are worth noting.
The new unit out by Backscatter looks very appealing too but I have not seen any reviews on it yet. With regards to the light on it I don't have a opinion as to whether it is better to be external to the housing vs. internal for the Housing Sentry nor do I have an opinion for the seal on the Backscatter system and whether its disconnect is any better than the stainless steel from the Housing Sentry. So far the Housing sentry has held up on over 3 dozen dives at recreational depths.
One feedback on the Housing Sentry is that many people prefer to remove the hydrophone since they don't use it anyway (and complain that the cord gets in the way). The Housing Sentry uses a differenct port. I personally like the option of having a mic available but honestly have not used it yet. I will when I get into experimenting with video this year.
One thing not mentioned in any discussion that I have seen is that for those individuals that have masks with the colored filters on them will have any difficulty seeing the blinking green or red indicator lights for either vacuum indicator. I know that the green indicator light looses some of its color in the Housing Sentry when in the water. While I still can see it easily blinking underwater it definitely is not as green underwater. There is a concern that if I were to slowly lose vacuum and the color shifts to red that I might not be able to pick it up as readily. I don't know about the Backcatter light indicator and if it does the same thing with the same colors. Nor have I used a mask with a red filter on it and if that would disrupt seeing the change in indicator color should it shift from blinking green to blinking red if vacuum were to decay off due to a slow leak.
The last question is just how much vacuum should one pump the housing down to and whether or not it is actually good for the seals? I have not seen a figure on what vacuum is "enough". Surely some of the vacuum must be pulling somewhat on the rubber o rings and will drawing a vacuum cause them to unseat or distort over time with the added sea pressure acting in the same direction?
One final note on using a vacuum system it is nice to know that having a vacuum will help keep the housing closed in the event that a latch were to accidently come loose or not snapped into place at all before or during a dive. I did try to pull apart my housing with the latches not latched and the halves would definitely NOT come apart. However, I did draw a pretty high vacuum when doing this (15" Hg).
My two cents.