I got the Firefly DVDs blind - I'd never seen it before, I just got it because I'd heard it was Travelleresque.
My first impressions weren't that good - Mal and Jayne initially pissed me off no end. Also, there is a very strong 'western' element in the first part of the series, which I didn't like much. Basically, the core worlds are high tech (but high society and clothing styles is kinda like 19th century Earth), and the frontier worlds are low tech and all look like the wild west for some reason. It's a bit of a mish-mash of styles that takes some getting used to. There are obvious parallels with the wild west elsewhere (the Alliance is obviously the equivalent of the Union, while the Independents are the equivalent of the Confederates).
However, it did pick up and become more sci-fi. I'm not really sure how Travelleresque it is in practise. The initial episode has them picking up passengers and cargo, but then they seem to spend the rest of the series "doing crime" - running somewhat dubious packages and making under-the table deals. That said, the ship itself is very much the equivalent of a Far Trader in Traveller.
Where it really shines is in the character development and the backplot (which sadly we'll never find out about). Mal does grow as a character and becomes less of a prick and more humane, Jayne goes from being obnoxious idiot to thuggish comic relief, and the relationships between the other characters gets more involved. Simon and River are especially interesting, but they're basically what the whole plot revolves around. For me, I think the turning point was the "Ariel" episode, though some of the earlier episodes (especially "Bushwacked") were good.
So yeah, it's a bit western (some are really obviously Western stories, like The Train Job and Heart Of Gold), but the sci-fi side is really quite good.
Don't expect another Buffy or Angel though - the style is quite different. The characteristic humour pops up more later in the series, but the start is a bit dry. One good thing about the series though is that it does actually remember what happened in previous episodes and you have recurring characters.
Speaking as someone who was completely new to it, I'd say grab it and grit your teeth through the parts you don't like - it's worth it in the end. Then watch the special features on the last disk and start wishing all manner of gruesome death on the idiots at Fox for killing the series practically before it started.