We used it last year to get the the Hualumpong Railway Station.
It was smooth, CLEAN, and NOT like london underground.
Plenty of trains and I think it was cheaper than the Skytrain. The route is different.
You do not have a travelcard, but a round silver token.

It is a bit more limited than the likes of Hong Kong but I think they had a lot of problems. However, if it makes travel easier then its all good

I use it whenever I can in BKK... it is fast, spotlessly clean and generally less crowded than the Skytrain.... and the aircon is to die for on a hot day. In fact I am thinking of using it today to go from Sukhumvit to Chattuchak to the market... once the rain stops that is.. been raining a lot this week.

I've just posted a couple of pics of the underground on the photos section.

I didnt have any problems with it. It was clean, fast and efficient and the only reason why we limited our use of it was that many stations can only be accessed through long flights of stairs, which when you have a couple of buggies can be difficult to negotiate.