We're going to Stockholm end of May and want to take a side trip to the Carl Larsson Museum in Sundborn (just outside of Falun). Has anyone been and do you have any advice if it would be a pleasant drive from Stockholm (about 3 hrs) or if a train would be best?

The Carl Larsson home and museum in Sundborn is a charming and fascinating place to visit; I’ve been there twice. It's eight or ten miles northeast of Falun so if traveling to Falun by train, one would still need to take a taxi or local bus to Sundborn. Local bus service is intermittent on weekends. Trains between Stockholm and Falun are fairly frequent, about 12-14 daily depending on the day of the week, with departure times between about 6 AM and 6 PM in either direction. In many cases there is a change of trains in Gävle, north of Stockholm, but this doesn't appear to have an effect on total travel time of about 3 hours. Fares seem to vary quite a bit depending on time of day, day of week, and specific train. Fares in May 2005 run as low as about $30 (second class, adult, one-way) to about $115 (first class, adult, one way). Second class is a perfectly acceptable choice on Scandinavian trains. The travel time by car is about the same as the train ride. I have made the trip between Stockholm and the Falun area by car and recall it as an easy and pleasant ride.

www.clg.se
www.sundborn.info

There are other things to see in Falun and the surrounding area. Falun is the site of a giant open-pit copper mine known as Stora Kopparberget (the Great Copper Mountain), which operated for nearly a thousand years and was once the single most important element in the Swedish economy. One can visit a museum and a portion of the mine itself.

www.visitfalun.se/english
www.kopparberget.com

If you're in the area you should consider visiting Leksand and Rättvik, two small, picturesque cities on Lake Siljan, not far to the northwest of Falun. The whole area is rich in local heritage and folk traditions.

www.rattvik.se/in_english.asp

Northwest of Rättvik is the small city of Mora, the home of another well-known Swedish artist, Anders Zorn, and another museum you can visit.

www.zorn.se/engmain.html

And to the northwest of Mora, in the tiny town of Nusnäs, one can visit a factory in which the famous carved wooden horses, the Dalahäst or Dala horse, are carved and decorated.

www.nohemslojd.se/index.html
www.grannas.com/eng/index.htm

Tourism sites for the general area:

www.siljan.se/ny2004/eng/index.htm
www.sodradalarna.com/ENG_index.htm
www.dalarna.se/default____4068.aspx

As you can see these suggestions lead increasingly far afield northwestward from Falun and Stockholm. If you plan to visit only Sundborn, the train is certainly an option. If you should plan to visit any of the other areas a car probably becomes necessary for any kind of efficient travel.

Good luck.

I can't really give any good suggestions for routings to get there unfortunately.
However I can say I was there a few years ago and it's a lovely place to visit with great surroundings. If you have seen some of the paintings of Carl Larsson you'll recognise the buildings.
Take a guided tour and you'll hear everything worth to know about the place, the guides are very friendly and enthusiastic.
If you go Dalarna (the county where Sundborn is located) do allow some time (other than visiting Sundborn) to see the surroundings as it's a region with many old traditional Swedish houses and villages. There is also a nice quite large lake called Siljan.
This is also the region where they are (probably) most traditional in Sweden, many people from Stockholm for example go here to celebrate midsummer as you get "the real experience".

I would think the end of May is a good time to go there. Acceptable temperature (around 20 degrees) but before the major public holidays starting in the end of June, ending in the beginning of August. At that time it's probably very crowded there.

Thanks to both of you for the suggestions and advice. We decided to add a few days to our trip and drive up to the Dalarna area - staying at a little place that looked charming (Akersblads - have you seen or heard of it?) and will work in some of the places mentioned before we go to the Carl Larsson museum and then head back to Stockholm.