Also, why it is only decent? The Prague mass transit system has comfortable clean and warm vehicles, extremely good coverage (14 commuter train lines, ~24 light rail, 3 metro and hundreds of bus routes), short intervals and all of that is also in the suburbs. I have been to a lot of places, but haven't seen a system that functions much better.
Thank you, it's nice to hear this as a resident. Alas, I'd also go for "decent" because although the transit system is good, it's being sabotaged by the city council - it prefers to fund building highways through the city center, and to sell off the profitable parts of the public transit company. Dark times are ahead, what with much of the rolling stock nearing the end of life and much-needed extensions being deferred indefinitely for purely pork-barrel reasons.
Oh, and it costs <20EUR per month...