Trains, Taxis, and Chaos. Very Mexican. Very Fun.
I had the advantage of going with students from a local spanish language school I was attending. We planned to go to Xochilmilco for one of the women students' birthday. I had to attend church that day and almost missed a Great adventure.
As of August 2008, the Xochimilco Train Station at was being renovated so the train stops one stop early at Francisco Goita Station. Since I had limited spanish skill, the taxi took me to Embarcadero (Belen) which was dirty, and located in an area that looked like an old neighborhood. It was a little spooky. My student friends were not there so I took a taxi back to the train station with a random drive-by of the station we were originally supposed to meet at.
Fortunately, the birthday girl was late and I luckily connected with our (90 minutes late) group at the closed Xochimilco Station when I gave up and was returning to DF. Then the true adventure began.
The basic atmosphere was of a VERY crowded waterway. The sounds of Mariachi were everywhere. Performing mariachis, drinks and food can be procured from your boat from various floating vendors.
My advice is to go with a group (it is alot more fun. Think party/fiesta) and with someone who is Mexican (if possible). Also, buy food, beer or sodas from the passing boats. It was all good so you will like it.
Also expect the river, like all of Mexico City, to be crowded and full of music. If you don't like crowds, neither Mexico City nor Xochimilco is the place for you. I have no problem with crowds and I loved it! I'm not the boating or romantic type but I give high ratings. However, those who want quiet and an isolated boat ride will only see a tourist trap.