Originally posted by lastdodobird or maybe I should just get the Sigma 18-200 and skip both the 17-70 and 70-200?
While the extra long zooms are convenient, you lose IQ since they have to pack so much into one lens. I think that the 17-70/70-200 is the way to go. You can pick up the 17-70 for $350-400 new and around $300 gently used, which is very reasonable for a lens that's this versatile. While the 70-200 is going to be the best in terms of IQ, it is expensive, whether you get the Sigma or Tamron version. If you don't want to spend a lot of money at this point, then picking up an older F70-210 like I have is a good option. Speedwise, it is slower than the 70-200, plus it has a fairly stiff zoom ring and loud AF, but it produces some really sharp pics. You can usually find one of these for $200 or less.
If you decide that you must have an extra-long zoom, go for the 18-250, either the Pentax or Tamron version, as the IQ is more highly regarded than the 18-200.
Again, HTH,
Heather