Well Mike, since i'm the one that showed them to you, maybe you're looking for another opinion, but just for the record, here's mine:
These retrofits are great for planted aquaria, where the intensity of lighting and the focus of the reflectors is essential to power the light down to the plants. They are also great if you want to be able to access the terrarium without removing the lights.
However, in terms of overall lighting, i think you can probably do better with 4 F32T8s, at a fraction of the cost. The light will be more uniform, and the lamps much cheaper to replace. If you want an easy, cheap, efficient light, go with the HD shoplite special. If you want to build a custom hood and/or are willing to pay more for easier access to the terrarium, go for the ahsupply retrofit.
I should mention that the retrofit isn't very useful unless you have a 4' lightstrip to gut out, or are building a custom hood.
One last issue is what it is you plan on keeping in the terrarium. My 55G holds just lowland Neps and tropical utrics and Queensland sundews, all of which do excessively well with just two F32T8s (and "survived" for more than six months with a single one, as an experiment). It would be a serious overkill (perhaps literally e.g. for the N. northiana!) to put those PCs on top.
OTOH, if you're trying to grow highlands in the terrarium, a compact, high power light source is in order, and has the added benefit that it's easier to actively cool to keep temperatures down and leaves room for ventilation out the top of the terrarium.