Not sure if the 50-200 is good, but I've been using a borrowed copy of 70-300 macro version for a while now, mainly for macros, I like the range and the fact that it -is- macro, albeit fake-macro and 2:1 ratio.
A little soft on the 200-300 end yes, but I've taken a few potshots from 70-200 and it's pretty sharp. Good if you like to take butterflies and dragonflies, and don't need those longko-style large large bug eye shots.. =D Would post a few pictures, but sharpness issue not that visible at this size anyways.
I have been using the Sigma 70-300mm for the last 6 mths when i bought it. Its a good lens but its not as sharp as the others. The good thing about it is that its more compact and lightweight than Similar nikon or canon lenses.
This the lens I use when I need to travel with minimal gear for work. Great Shots and it lets me even take MACRO at 200-300mm. It has never let me down yet.
However the Zoom ring gets abit loose over time. When it was new it would not extend out by itself if you held it upright but over time it thens to slip out by itself.
Anyway overall good lens, quality good, size & weight very good, made in JAPAN and best of all price point; less than S$350. MS colour selling at S$330.
Using telephoto lenses one has to give up something over the other.
Prime lenses (which are fixed 35mm or 50mm, etc) usually give the sharpest and clearest shots.
Telephoto lenses usually sacrifice some sharpness and clarity and sometimes there are some aberrations (u are lucky is it is not that noticeable, especially on good quality lenses).
However, the biggest plus for tele lenses is the fact that they can help you take shots of different focal lenghts.
The actual truth is today photo bugs are really really spoilt for choice! I remember using only 35mm or 50mm lenses to take shots. Long focal lens or telephoto lenses were really astronomical in price and difficult to get.
So up to you, choose well. And for your info, some of the best photos in the world were taken
by prime lenses 70mm or less, usually 35mm. Think about it.
for travel, i think the 18-200 has a very versatile range without changing lens
but i am thinking of getting better quality lens (without bursting my budget) when going for dedicated photo shoots. 17-70 is the main reason for changing my setup, 70-300 is to cover the remaining range