This is an interesting discussion - it's good to hear from different people about how they see this issue.
Personally, I think the super-low price of the PT house is a good thing all the way around. It gets people UW with a nice 5MP camera that can take great images. The biggest down-sides you face is the low-durability/short life-span of the box and no hard-wired strobes will work with it. (The housed Oly strobe being an exception with the PT-020, but it's not the greatest...) On another level, these housings are very nice. The camera's do have shutter lag, but their lenses are super versatile: you can go from WA to macro in the same dive. (Even more so if you have wet-change-able lenses.) They are also very compact. As a learning tool, they are ideal. You get started for cheap, and can take great images right away. Particularly for people who dive only occasionally, the PT houses make great sense. It a great option for starting out.
As soon as you start adding strobes, things kinda change. I think the most salient difference in the house you choose, beyond durability, is the strobes you can use with it - and how they will work. Inon has a wonderful strobe for use with the PT houses. The D180 can do excellent auto exposures with it's fiber-optic system. (It takes-over the pre-flash function, giving you accurate D-TTL.) The D180 costs as much as the camera, however, and you can't hard-wire it - so to use it you will always have to have a clear plastic housing. If you go with the Inon Z220 you pay more AND have to give up D-TTL, but it will work both ways (hard-wired and fiber-optically).
On the other extreme of the price scale is the Tetra for the 5050/5060. While it's not entirely accurate to compare only the list prices (street prices are lower) six to eight times more expensive is about right, depending where you buy. The Tetra is metal, very durable, and has circuitry built in allowing you to hard wire the sync cord and get perfect D-TTL exposures with many UW strobes - including the Z220. It also has a leak alarm. The strobes are controlled from the housing - no reaching for the strobe - very nice indeed. As other have said, LOTS of people who carefully compare the features and value between the PT house and the Tetra pick the Tetra.
My first camera was a Nikon CoolPix 5000 and a Tetra box. It didn't do good D-TTL (like the 5050 and now 5060 does) but I used it on hundreds of dives and it took great images. The control features of the Light & Motion housings are a cut above the rest. Aquatica makes a great metal house for the CP5000 - for MUCH less than the Tetra - but you don't get some nice control features the Tetra provides. (On the other hand, if you use a dome with the Aquatica, you can use the Nikon 19mm WA. For many, this is THE nicest setup available in a "point & shoot" camera today.) The Ikelite house for the CP5000 is a couple hundred less than the Aquatica - but of course now you are talking about a plastic house. (Not that it's bad at all - but it IS bulkier.) It's only my opinion, but I don't think the PT holds a candle to any of these - but as an inexpensive start, by my way of thinking the PT wins the day.
Personally, I think the days of the P&S digital cameras are (for many shooters) numbered for UW. There will always be people who want the compactness and versatility of the P&S option, but with the new Canon D-Rebel and Nikon D70, a new day is dawning in UW digital photography. For a relatively small increase in price (over a 5060 in a Tetra house) you can now be light-years beyond any of the P&S cameras - just take a look at the D-Rebel and the Aquatica A300 with its E-TTL! The exception is the 5050/5060 & PT. As much as I admire the Tetra, for my money I'd spend a little extra and get a DSLR - or I'd spend a LOT less and get the PT & 5050/5060. Note I say "for me" - for others the size/portability and lens issues might hold sway. My advice is to take a serious look at the DSLR options before deciding to spend $2000+ on a 5MP P&S camera and a house...