Mine arrived from Star yesterday, and he's mostly a great little figure. The vehicle mode detailing is excellent, it feels good, and the Slimer is a good sculpt. The fact he's compatible with the Siege Blast Effect ports is a plus.
The downside is the robot mode. It's not bad by any means, but it is very reminiscent of the Star Wars Crossovers range. In that it feels like they've focussed on getting the vehicle mode right, and not worried so much about the robot mode. Thankfully, being the mold it is, the transformation isn't as bad as in those lines. And really it's just the head sculpt that's a let-down, being rather plain - sure, they got goggles on him, but forgot about needing to mold the rest of the head.
But that said, it's not a massive let down - it's still a robot with a proton pack that turns into Ecto-1, and as such it's pretty great.