Interesting article from LOWES on gutter replacement:
The rules of thumb regarding gutter replacement are simple:
1. If your original gutters are straight, solid and don't leak, they don't need replacement, period.
2. If they do leak, there's a fair chance they can be repaired. In the case of steel or copper gutters, contact your local sheet metal shop. For redwood gutters, have a good handyman determine if they can be caulked or patched.
3. If you do decide on replacement, demand gutters that are at least equal to the originals in quality. Ask a knowledgeable but disinterested party (not the roofer doing the work) to recommend the best material.
4. Lastly, put at least as much thought into choosing the gutter profile (the cross-sectional shape) as you do into choosing the roofing material. Don't let the contractor make this choice for you; many will simply fall back on the style of gutter that's the least trouble to install.
Go to http://lowes.inman.com/newsletter/2012/06/05/news/190037 to read the full story.