Can you fly in clouds?
Yes, but it requires a filed flight plan and extra pilot training. I received this required extra training, known as an instrument rating, last year. A flight plan is required in this case so that air traffic controllers can keep airplanes flying through clouds separated from each other.
Can you land at SFO?
Yes, but I don't think they'd be very happy about it. Large aiports often charge large landing fees to discourage smaller operations from clogging up their airspace. But, if I really wanted to, they would allow me to land.
How much did your airplane cost?
That's like a car!
Exactly. And aside from the operating costs, the maintenance costs aren't too different from that of an aging german car. (I also own one of those. It provides much less enjoyment per dollar.)
Is it cheaper than commercial?
Not even close.
Flying small airplanes, while sexy, fun, and fufilling, has a hard time competing with the airlines. The economies of scale you gain from shoving hundreds of people in an aluminum tube generally keep commercial flying much cheaper. That said, if you split the fuel costs with a couple of other paying adults, the costs can be comparable. I did a round trip from San Francisco to Vegas with a couple of buddies, and the split on fuel came out to around $150, although it did take almost 5 hours each way.
Are you allowed to fly anywhere?