I'll begin by stating that I I don't support such a mission, as I prefer robotic exploration. But this proposal isn't as extreme as it may sound - it's probably a heck of a lot easier than landing on a planet and taking off. It's only 640 m/s from earth escape to Venus (3/5ths that of Mars). Transit time is less and launch windows a lot more frequent. Venus offers very easy aerocapture. You don't have to deal with the randomness of the surface - your "landing" is a lot more forgiving. Your habitat is probably simpler, not having to deal with a surface (although there's a few potential complications that need to be studied, such as storms, and I don know the radiation level at the desired altitude). Keeping it aloft is easy - even normal earth air is a lifting gas on Venus. Solar energy arriving at Venus is double that of Earth. Nearly earth's gravity eliminates a lot of the uncertanties about skeletal and muscular wasting.
One of the neat things is that a person could potentially step outside without any sort of special suit, just an oxygen mask. It's a "maybe", though, as there's a few complicating factors. It's 37C (100F) at the same sort of heights that it's about 600mb; for US analogies, it's Phoenix temperatures at Mount Whitney air pressures (lower or higher for both, depending on your exact altitude - you can choose). So it's not a perfect match - but probably tolerable. But there's two potential complicating gases: SO2/sulfuric acid and carbon monoxide. Breathing them is right out, but even long-term (hours at a time) skin exposure might be problematic at the given concentrations; it's not certain whether at these altitudes they'd be prohibitive. They would however make eye protection a must at the very least, the eyes are more sensitive to both CO and SO2 than the skin.
Manned or not, the main advantage of a Venus blimp would be the lower altitude it would provide to scientific equipment versus satellites. So you'll get a lot more information on the atmosphere, which could help answer questions about Venus's evolution (and how other worlds in other systems might be). You'll get higher resolution radar imaging of the surface. You simplify to some extent sample return missions from the surface, as each sample collection doesn't have to be a self contained return mission. Etc.
One thing on Venus I'd love to see studied more is the super-reflective radar surfaces. It's now believed to be due to a "galena snow", snow made of shiny, electrically conductive lead sulfide. I'd really love to know more about the surface minerology of Venus in general.