Is there any disadvantage in getting USA agent?

I would get an agent in the country for the market you are trying to sell to, they will have the best contacts.

If your book appeals to US fantasy markets then you should find a US agent I believe.

Afterall, it doesn't matter where you live but what market your book is targeted for.

I live in the UK and have a US agent. He also represents the likes of Terry Pratchett. So, obviously, it's no problem at all.

I know several UK SF/F writers with US agents.
The only disadvantage - they are less likely to sell your book to a UK publishers so you won't see it on the shelves of Waterstones

... if they charge for phone calls (a few do) you'll be out a few pounds each year for the long distance charges. And of course you'll have to pay extra yourself when contacting them by phone, if you need to. Not much of a disadvantage, to be sure. Will try to think up something better ;-)

Even that isn't a disadvantage with Skype

... wow. Neat service!
The phone companies must love Skype ;-)

It's always best to have an agent who routinely sells books to the giant publishers, wherever that agent lives.

If you are considering US agents I presume you are aware of this
http://www.absolutewrite.com/forums/showthread.php?t=42019

OP - one advantage of querying US agents over UK agents is that most nowadays accept electronic submissions. So instead of spending money on printing and posting all those cover letters, synopses and sample chapters and waiting several months for a response (if you get one at all), you can just send a query letter. Only downside is you need to spend a good bit of time getting your query letter in shape, it is a lot different to what you might send to a UK agent, but there is some great info here.