Originally Posted by
RobertsMx
I can't speak for the 18-200 (compared to other 18-200 lenses), but Tamron 90mm f/2.8 is a fantastic lens. I see no point in replacing it with another similar macro lens. The Sigma 105 is also a very good lens (I have Sigma 70/2.8), but it is more likely to be a lateral move than an upgrade, except for HSM and OS (neither is very useful for macro).
As far as 18-200 is concerned, if you're dissatisfied with the IQ, and know the alternative is better, then it might make sense. I wouldn't go simply by brand name though. If you feel 200mm is limited reach, then that could be another consideration. I use Sigma 18-250 HSM OS on my Sony A55 (can also use it on my NEX-3) which does its job well, but if I wanted to really care about IQ, then considering a faster prime or two for the cost it might take to replace the two with similar lens may be something to consider (example: adding 35/1.8 or 50/1.8).