Paying the first month's rent and security deposit is one of the most important parts of the apartment hunting process. It's tangible, monetary evidence that your relationship with a landlord has begun.
Typically, the first month's rent and security deposit are paid even before you've moved a single box into your new apartment or hung up your favorite photograph. Because of this, don't willingly hand over large sums of money solely based on trust. No matter what the landlord says or how nice they seem, always hand over the rent and deposit checks only after you've signed the lease and received your own copy.
Some landlords are lazy about the lease. Maybe they're new at it or it's not their primary occupation. Whatever the reason, these landlords accept your money and let you move in. They say they'll send a lease over later. Everything is easy and casual, and the apartment is great. But then something happens, like your refrigerator breaks down, and you realize you have yet to receive the lease. Now, it's a sticky situation, because you have nothing on paper stating what the two of you have agreed to. No matter how nice or trustworthy a landlord may seem, when money is involved, people protect themselves first, and it can become a contest of he-said-she-said.
So, no matter how nice the apartment is, always be prepared to walk away when there's no lease to sign or you don't get your own copy at the time you pay the first month's rent.