I love my MPK-P9. I use it with my DSC-P9 when snorkeling and it works GREAT!! It is easy to use the LCD viewer once you get accustomed to the light under water. I have only one concern: Sony changed camera models and now this underwater case is basically obsolete. If I have trouble or need replacement parts, I'm out of luck!!

I was a little skeptical about putting my $500+ digital camera in one of these marine packs. I actually own the Cybershot P1 version, but my fears were unfounded as long as you follow the instructions.
The marine pack comes with a grease for the O-ring (yes, everything depends on the O-ring working) and the O-Ring. With the edge of a business card, the O-Ring comes out of its seat and can be cleaned, I use a lint-free paper, like lens paper. Then the grease is applied (I put it on the tips of two fingers and work the O-ring through to grease all sides). The O-ring is then placed back into its channel. With the camera inside (the pain is you have to take off the carrying strap each time you use it), then the pack is locked down (can't be opened without a second button moved) and the O-ring seals the back of the unit against the frame.
I have gone down to 50 feet without any problem and used in the open ocean as well as in the reefs (great in the pool too). Once you are a couple feet into the water, don't forget to use your flash.
After a three-week trip to Aruba we brought back 700+Mgs of pictures (dumping them into the laptop daily). If the picture color seems off a little (dull, washed out), use the Sony Picture Gear software that comes with the camera and use the Filter - Color Balance Adjustment. It will greatly enhance your photographs and give you eye-popping pictures.
It is always important to clean and re-grease the O-ring before every dive. It is smart to pickup an extra O-ring for [cheap] so your trip won't be spoiled if your O-ring is damaged.

I found this housing useful for shooting photos and video at the beach. However the latch seems weak and has already broken once on me, and since damage isn't covered under warranty you'll be out some cash.

The Main Problems:
You can't turn off and on the LCD without opening the unit which runs down the battery.
The lens fogs up after about 45 minutes of use in and out of water.

The Sony P5 or P9 with the Marine Pack are an amazing combination. I have done dives to 130 feet with no problems. It is important however to assure that the camera is turned off underwater when you do not use it, otherwise the heat will cause fogging of the lense no matter what you do. The video is quite amazing when you consider the size of the package. I still would wish a Marine pack for my Sony DV camera, but this is the next best thing. I am also looking for a red filter as Sony had one for the P1. I usually carry the camera in the BCD pocket, and do not bother to handle it especially careful. Extremly rugged assembly. Any dive has just become a photo and video dive with no extra hassle or worries. One word of warning: If you dive with a camera, you will sink (flood) one at some time and be 500 bucks poorer. Count on it! Mr. Murphy calling ...

I bought this Marine Pack to use with my DSC-P9 while snorkeling in the Carribean. It works well underwater and takes very clear pictures. A few things you should know before buying this unit however:
* Not easy to use. Requires special grease (included) on the o-ring to keep a water tight seal.
* Requires anti-fogging solution (included) to be applied to the inside of the glass. Even with this solution, the glass fogged after an hour or so.
* If you pull the unit out of the water to take a above-water photo, it is very difficult to dry the glass, and any water droplets really blur the picture.
* The optical viewfinder on th camera is unusable when used inside the marine pack. The only option is to use the LCD viewfinder, which is impossible to see on a sunny day. You just have to point the camera in the general direction of your subject and hope for the best.

Overall, this unit is the best way I found to take a digital camera underwater, but there are a few gotchas as mentioned above.