So unless you have significant development resources and you need to scrape just a handful of sites, you may be better off to see if there are any companies/open source projects that have already learnt the lessons and fought the battles of web scraping. That way you can concentrate on your core competencies.
By the way a company that adopted this strategy fairly successfully is mint.com. They used Yodlee to do the web scraping for them. This was circa 2006 both companies seemed to have moved on since then.
You might want to look at this discussion http://www.quora.com/Web-Scraping/What-are-some-good-free-web-scrapers-scraping-techniques
Finally, in my opinion, the hard job in web scraping is to handle the idiosyncrasies of various web sites so if there is a great non-RoR tool it may not be that hard to integrate it with a RoR back end to handle the results of scraping.
Best of luck,