I must say there is not any negatif affect if you use GWT...

It's will be easier for google bot to index our site when we use it..

The ones that affect SEO IMHO is backlink,onpage and offpage optimization..

just it...,nothing else

Cheers,
Vaan

I have heard a lot of conspiracy theories about this.
In my opinion it does not matter. Why would Google penalize people that use their tools?
If anything I would think it would be the other way around.

"this guy's a dumbass for using our tools..........penalize him!"

Doesn't make sense to me...

Just because your Googlenoid doesn't mean that they're not out to get you.

No I don't think they will penalize you for using their tools.

The use of WMT won't directly affect your rankings or PageRank. However, using the tools inside of WMT's can help. You can ensure Google has your sitemap configured, check the broken links reports, configure your site for a region (ex. US only) and see some of the links/keywords you site is registering for

Google Webmaster Tools are useful for purposes of submitting Sitemap,robots.txt,broken links stuffs and more.Checking Broken Links,Making your site indexable easily with the help of sitemaps are also considered a vital SEO aspect.

Hence,Even though Google Webmasters tool is not directly related to SEO,it is indirectly one of the aspects of SEO.Sitemaps,robots and broken links are directly related to SEO.If GWT is related to sitemaps,it is indirectly also related to SEO.

Using GWT tool will only help your cause and it won't hamper anything(Not counting the people who didn't know what to do but still messed up with it).

Hope i was understood well.

I'd imagine using google products would only help. After all, you'd be supporting Google.

That's, at least, how it should work on the surface.

The Webmaster Tools are great for finding the keywords that you aren't directly targeting (the high traffic 'short' tails) but are creeping up the list on. Look for words where you are appearing regularly but on the 2nd or 3rd page as you won't be getting any click throughs but you will know that the word has the traffic for a percentage of people to go that deep into the results. You will find some real gems.

No its not affect anyway

GWT is usefull product of google to SEO.

I don't think so...

I am not using Adwords for my site - will it going to affect my ranking? - 'Big No'.

I trust Google that they not even think about issues like this.

I use it myself i just think its a rumour nothing else