Don't let other reviews fool you, the heel works just fine. I was initially worried as the heel fit kind of funny, but after some use I would definitely say I like the heel better than my old la sportiva miura vc. The booster's heel is the most sensitive I've climbed with and it molded to my foot with more use. The rest is just as awesome, the shoe's design forces your toe into a position that really creates a powerful boost when you need it.
Break-in wasn't that bad. The first few days of climbing were tough but not terrible, unless you're not accustom to a down-turned design, then they might seem pretty rough.
Sized one size down from my regular street size, I got a size 42 and my street size is a 43. I've got a medium sized foot as it's not too wide or thin.