It's a lot harder than learning to play the guitar, I can tell you that much. I'm trying to teach myself to play the piano now, and it's friggin' ridiculous. I'm teaching myself Clair de Lune, which isn't even all that complex, and am still having trouble. So yes, I think it's pretty difficult if you don't take lessons or start early.
The basics no, however for more complex techniques and to help teach you how certain notes works, you'll definitely need some type of lessons. I've been playing piano for 15 years and for most of that I've had lessons, they helped a lot and it gets you good faster. It is however very possible to learn it, but it will take some work.
It depends on how far you want to go with it.
Most people can pick it up well enough to plink out simple song arrangements. Getting good enough to sight read or play with a band is a lot harder.
It's interesting. I'm a piano player and cannot grasp the guitar. I think it depends what you're used to.
well i cant read music at all!! but i can learn some songs on utube, using my grans keyboard
i think its one of the hardest..... and it takes quite some time to learn properly