I'm looking for a decent all-around lens and I was planning on buying the Canon 18-135 EF-S lens but don't know if I should consider something else, perhaps something from Sigma like the 18-125 or something else altogether.
Any opinions?
Take a look at sites like photozone.de,
http://www.imaging-resource.com/ , and fredmiranda.com for opinions on comparitive lens sharpness and imperfections. Look at pbase.com where you can search for shots taken by a specific lens (list of lenses are in the search by camera section) and decide if the 18-135 is good. "Decent all-around lens" is too vague without a budget number.
If your budget is around $400 to $500 (firm), I'd consider the 18-125 or 18-200. I'd go for focal range convenience over all else. I'd crank up the ISO and stop down whenever I feel I need better sharpness in the corners (don't always need it). Above all else, I'd learn what is good enough for my current needs. If you share photos on the web or print 4x6 or no bigger than 8x10, you don't need the ultimate in sharpness. I would then buy and learn to use decent post processing software and get an external flash and learn to use that. You'll photographic skills will grow with such an approach before you get caught up in buying $700+ zooms or L glass.
In that respect, if I did it all over again on a tight budget (I started with a 300D, the first 18-55 kit, a 50/1.8, and a 75-300 II), I'd stick with the 18-55 IS kit and get a 55-250. I'd then go for an external flash and learn how to use it.
If I had an extra $700, I'd replace the kit with a 15-85 and still get the 55-250.
I never tried the 18-135, but, I have used the 18-200. The 18-200 is OK, but, then I'd work around its limitations (which is half the fun/challenge).