I have the Pentax FA version of the Tamron 18-200mm that I picked up for very little money.
The focal length was versatile for 35mm film but on digital, there is not enough at the wide end to be useful for events. This applies to the 28-75mm as well in my opinion.
Overall I'd rate the 18-200mm as an average performer typical of superzooms. Expected distortion at the wide end but performance starts getting better up until about 135mm, it gets to be soft beyond that. This lens is a best used in bright outdoors.
Some sample images:
BTW this thread should be in the SLR Lens discussion section.