My last trip was a game changer thanks to a decent pocket-sized paperback. Slid it right into my jacket alongside my wallet, and it was there for every idle moment—waiting in line, on the subway, during lunch. The freedom is huge; you're never stuck without a story. I find I actually read more because the barrier is so low.
That said, there's a trade-off. The text can be annoyingly tiny, and the spines on those mass-market editions crack if you look at them wrong. And forget about any serious maps or appendices; they're usually stripped out or rendered useless. For a quick, disposable read, it's unbeatable. But for a book I want to live with, I'll take the full-sized edition every time, even if it means carrying a bag.