I recently built and painted some of Rubicon's US Army & USMC Vietnam War plastic miniatures for Goonhammer.com. You can check out my review, comparison, and painting methods
here.
The models are a bit fiddly, but I'm still pretty pleased with how they came out.
I have to say I think the USMC set is the better of the two, as the models require less assembly and come with far more weapon options. I thought it was odd that the US Army box didn't come with M60's or any other machine guns.
Next up I'm working on their Huey, Viet Cong, and North Vietnamese Regulars.
If anyone has Vietnam minis from other manufacturers I'd be interested to hear how they compare - both in terms of scale differences, and quality!