I recently purchased a "Squirrel Buster" feeder over here. It is very adjustable via a twist mechanism for tensioning the spring inside the feeder. Other benefits include a ventilation system so that seed won't get mouldy, and an optional "Cardinal ring" for attaching to the perches. This is good because some birds prefer to be able to face the food, rather than having it be sideways-on.
In action, it does work, though it hasn't been seriously tested by my squirrels yet, since I feed them separately and they haven't felt driven to attempt access. However at the store where I bought it (Wild Birds Unlimited) they had a cap from one of these feeders hung up. It had been opened like a can by a squirrel, who had basically chewed through the metal all the way around the top. It was quite amazing. The store was making a point about their lifetime warranty, which means they replaced the cap. In any case, if this particular squirrel felt driven to go to that extreme, then I guess you could say the feeder worked! ;-)
I have found that birds such as grackles are able to use the feeder even when they shouldn't. I adjusted it so that the weight of a grackle would depress the spring, thus closing the seed doors, but these highly intelligent members of the crow family weren't phased one bit. He would simply sit there and stick his sharp beak into the top of the crack, and lever the whole thing open, take some seeds, eat them and repeat. Didn't seem to mind at all. So the thing worked, but since it was on a spring, he was able to circumvent it. I guess if you don't have these birds (I never saw them when I lived in the UK) then you will probably be ok - I don't think I have ever seen a starling doing such a thing.
I have no idea whether a squirrel would be able to find a way to hang down and feed upside down without depressing the spring; but I think that these do fundamentally work. Try to get one which has an adjustable spring tensioner, that will give you the ability to tune it for particular bird types by weight. It is a cool invention, definitely not just one of those "As seen on TV" scam inventions.
