Car batteries are definitely a no no. Unlike SLA batteries, they are not "sealed". They are also not designed for deep cycles. Car batteries are designed to provide a lot of amperage over a short period of time which was needed to turn the motor and get it started. A car battery can discharge very quickly on a heavy load. If you try to start a car and it takes more then 30 seconds in one start attempt, the battery is effectively dead at that point and would need to be recharged. (that's what the alternator is mainly for on most vehicles. It charges the battery after the car is started)

They can be used in the same manner as SLA batteries, but don't expect it to have the same discharge characteristics that SLAs have (SLA = Sealed Lead Acid). That and deep cycles will damage car batteries even more then they would SLA batteries. Not to mention car batteries are really heavy compared to SLAs. (and larger in size too)