Paul,
I honestly think that you'd be happy with any/all of the macro lenses suggested in this thread. Choose one and enjoy!
If you're looking for a bit more info, I personally like reading the reviews on The-Digital-Picture.com:
http://www.the-digital-picture.com/R...ns-Review.aspx
http://www.the-digital-picture.com/R...ns-Review.aspx
Many of the "cool" effects you see with macro photography are due to good camera technique, lighting, and being able to get close enough to the critters. Here are a few random macros I've taken with the Canon 100 mm macros (both IS and non-IS). They probably would have been identical with the Tamron.
By the way, flash is extremely useful for macro photography: if you don't have an external hotshoe flash, you'll probably be tempted in the near future if you get hooked on macros. In the mean time, you can do wonders with your internal flash (e.g. Google "diy macro flash diffuser").
Macro lenses can be used for portraits since they're telephoto lenses and extremely sharp. However, they're not necessarily the most flattering portrait lenses because they're designed for accurate reproductions of images and colours - i.e. it's usually not considered very flattering to show all the sweat pores on someone's oily, pale white, pimply face super accurately in a portrait
Good luck and have fun!
Cheers,
Dave