Ed hit the high and low points already, but i work in the hearth
industry and I am not a fan of vented or vent free gas logs, even
though im pressed to sell both types. The reason most of our customers
have been displeased with them is this...

Vented gas logs require that we disable the damper during installation
(by building code and product instructions) and this is a huge heat
waster as you can imagine. We give the customer an option to have a
chimney balloon installed, but that must be removed when the firepalce
is used. Furthermore, even while you are using them they dont give off
usable heat.

Vent free gas logs are a better option if you want to actually get
heat from the gas logs, but they do effect indoor air quality (no
matter what the manufacturers propoganda says). As Ed said they put
excess moisture in the air and they do out put carbon dioxide and
carbon monoxide. So if you have infants or pregnant women in the home
it is not recommended. Your oven says "dont use as a heating device",
likewise vent free gas logs are not a heating device they just happen
to put out heat. If you have an old leaky house it is not an issue but
if you have a newer efficient home the vent free gas logs are not a
great option.