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I think so for two reasons:

1. For humans, it is easier to read and to identify keywords

2. For search engines, almost the same as above - easier to identify keywords.

Nothing noticeable today but I've noticed some weird behaviour in the past couple of weeks. I actually contacted Google as one of the site I'm managing totally fell off the map. However, they wrote back saying that everything was normal and when I checked the site again, there it was on the first page. Odd.

Also, I think it is best to use dashes as opposed to underscores. One selfish reason is that I find dashes easier to input as opposed to pressing shift + dash.

I would use Google Webmaster Tools as you can see how many URLs are indexed based on your sitemap. Once you have that, you can compare it to your total list. The same can be done with Bing.