I have been several times but the last time was about 7 years ago, however friends have been since then and have told me not much has changed.
It's good to get a car and drive from Cancun if you are going to stay at least overnight at a hotel near the ruins. There are some cool ones there like the Mayaland, and the light show at night is incredible. I have been to the the english version of the lightshow and they are really strict about not letting people walk around. The Spanish version which is usually ealier in the evening, they pretty much let you walk around at night. There was a full moon last time and it was really cool.
One neat thing about staying near the ruins is that you can go see everything before it gets really hot and before the big tours arrive. Notice my photo of having the place all to myself, well not exactly...it was a senorita that took the photo.
If you are just going for the day trip, I recommend the bus because believe me you will be glad you did on your way back....when you are pooped from having walked all over the area. It will be hot as hell as well.
Also, you won't have to deal with the possibility of "damaging" your rental car and having to pay fees. Furthermore, that price probably did not include the insurance. Gasoline can be kinda expensive as well and there are not many stations on that highway.
Believe me, it's a long way on the toll road to Chichen Itza. Oh that's another thing! Toll charges will mount up to almost $40. Plus there really is not that much to see unless you take the old route which will take you about 5 hours.
If you are just going for the day trip, take the bus, get some sleep on the way back. Go party in Cancun at night. Another thing, the lunches are not too bad and they sell beer at reasonable prices.
Have fun!