It does make a difference. Whether it's worth it is 100% subjective. It is quite possible to game and enjoy yourself on some cheap no-name rubber dome/membrane keyboard, in fact I did so myself for years. But adding a mechanical keyboard was worth it for me.
Logitech have been really slow to come up with a mechanical keyboard, instead adding rubber dome/membrane keyboards with more and more features. But they did finally get with the times late last year when they released the G710+.
I don't think it's one of the cheapest mechanical keyboards around though.
Personally I grabbed a Cooler Master Storm Quickfire TK, which is quite affordable. Comes in three versions with different mechanical switches and different backlight colors. It's quite small, no macro keys, and the numpad shares keys with PrintScreen, Del, PgUp etc.
Matt S :
Just get a simple keyboard, gaming keyboards are just given a fancy name and they don't actually have any special features that would make gaming easier. Just get a cheap ordinary microsoft, razor is like the Alienware of keyboards & mouses.
Except some of them do. I agree Razer tends to be more flash than substance, but not everyone is that way.