I spent a weekend in Andorra in early Spetember. It was my second time there and I used a direct transfer mini-bus from Toulouse airport - a bit uncomfortable but what can you do.

While I wouldn't recommend spending a lot of time in Andorra, a day-trip from Barcelona is easily done. You have the options of taking a coach (i.e. bus) from either BCN airport or the city centre, there are several daily departures from each. Do note that the drivers don't like it if you want to eat something / drink alcoholic drinks on the bus. Just go to Andorra's official tourism web-site at andorra.ad and follow those links;
http://www.andorra.ad/ang/home/marc....orts/index.asp

Having said this, local people in Andorra tend to be obnoxious. Most of the people working in bars, restaurants etc appear to come from Portugal. However, a short visit to Andorra will be worth the effort.