In June I went to Kew Gardens twice, each time with a different friend visiting from the States. If you have even the slightes inclination to gardens or parks, this is a must see when you visit London. A key word of caution...guide books say you can see it in 3 to 4 hours, well I spent a total of 10+ non-stop hours over two days and saw most, but not all and my pace wasn't exactly leisurely.
Both times we took the train from Vauxhall in London and used the Southwest Trains summer 2 for 1 offer to get in, which given £13 for a ticket was good savings. It's just a short easy walk from the Kew Bridge train station. There's a nice pub with tables out front for a beer when you leave the park.On my first visit the focus was on the "glass houses":
I also did the
Treetop Walkway, 
the Davies Alpine House and the Rose Pergula. All amazing.The second trip we got tickets to ride the Kew Explorer (think DisneyWorld parking lot shuttle), which allowed us to see much more of the grounds. We visited:
- the Bamboo Garden and the Minka House
- the Pagoda
- the Japanese Gateway
- the Rhododendron Dell
Even though there are plenty of visitors and you certainly don't feel alone, you don't ever feel crowded...there's plenty of space. I really can't wait to go back and see it in different seasons.
My key word of advice is that the food is typical cafeteria food. There's no lovely place to sit and enjoy a meal and be waited on, so bring a picnic! And comfortable walking shoes.
