Crowds at Giverney

I was there 2 weeks ago and the crowds were very large, both the line to get in and the crowds/lines walking through both sections of Giverney (the pond side and the house side).
So this is one place where I would recommend you buy tickets online in advance and print them out at home. I did this and we walked right in. We followed Rick's advice and arrived just before opening time at 9:30am. To do this, you will want to take the train that departs Gare St. Lazare around 8:15am.
Even doing it this way, and arriving just before opening time, the crowds going through the pond and house were very large by about 9:45am, and really, really large by the time the first big bus tours from Paris arrive around 10am.
By buying advance tickets online and printing them out at home, and taking the 8:15am train out of Paris, what you avoid is the long ticket buying line. There's probably nothing you can do to avoid the crowds once you get in. The house was so crowded that it was basically just shuffling through in a line, which makes photos more difficult to take, if you like your pics without ten people in front of what you're trying to photograph.
Despite the crowds, we found the experience rewarding (but we are Impressionist fanatics).

I was there mid to late afternoon last July, parked the car in the half empty car park and walked straight in. It was fairly busy but by no means overwhelming, and we were there until closing.

We both thought it unique and enjoyed the afternoon immensely. I think the key is to be there when the tour buses aren't.

I just visited Giverny last week and actually followed a lot of Kent's suggestions:

• Purchase advanced tickets online; no printing required; I showed them the email with my e ticket
• Take the first train out of Gare St Lazare I believe is 8:20am
• Go straight to the lily pond which is the best part and work your way back to the main garden and house

That said, there were still huge crowds on the day I went. It was a Tuesday of fantastic weather (sunshine, temp in mid 60s). The crowds did turn me off a bit. However, it's also easily understood why people went the time/day I did. The flowers were in full bloom and weather was unbeatable. It's obviously really great time to visit.

So... is it worth the visit with the crowds, etc? I think it is, especially now looking through all the beautiful photos taken and fond memories of seeing the "real" garden and lily pond of the artist's paintings. However, I probably would not visit again from Paris city central. I'd much rather visit the garden on the way to Normandy with a rental car. It did take a lot of time and effort to train in from Paris. It's more suitable and efficient as a point of interest while en route to Normandy.

We go there a lot each year. We signed up for an art/photography program that allowed us to paint and take photos the whole day which was wonderful. It can get crowded in the summer. Get your tickets in advance and you can even sign up for a private tour.