Pro: Your vows will sound modern.
Instead of reciting words written in the 16th century ("for better for worse, for richer for poorer, in sickness and in health, to love and to cherish, till death us do part"), you can recite sentiments in your own voices that express exactly what you're feeling. You can get as specific and as personal as you want. It's the ultimate in customizing.
Con: Your clergy will likely want approval.
If you're marrying in a religious ceremony, you'll find that some clergy may give a thumbs-down to personalized vows, but it's worth asking what, if anything, is permitted. Others may allow them but ask that you include certain phrases. Civil ceremonies are like a free-for-all—you can pretty much do whatever you want.
Pro: Guests will really listen since it's fresh material.
Most of them have heard the standard civil or religious vows many times before. And while some folks will tear up at the sound of those traditional words, everyone will sit up and listen when they hear you expressing yourself in a way that they weren't expecting.
Con: He's just not that into it.
If he keeps putting off writing his vows, dude is sending you a message: It's not going to happen. But before you give up or have a major blowout, give him a couple of tips on how to get started, like asking himself what does he love most about you and why he's chosen you as a partner. He may also find it less of a grind if you show him some examples of other couples' self-written vows or suggest you write them together, rather than separately. Even though it'll be a bummer to hear his vows before your wedding day, you'll be less stressed knowing he finally got the job done.
Pro: Writing vows together is like having a great date.
To write heartfelt vows, you've got to open up and express your feelings, not always an easy thing for some guys to do. So this is a great opportunity to talk, just the two of you, and hear what each other has to say. Get a voice-recorder app to capture every word, then use those words as the basis for writing your vows.
Con: He wants to make his vows jokey but you're shooting for sincere.
Unless he'll sound like he's auditioning for a spot on Jimmy Fallon's writing team, there's nothing wrong with letting his humorous side come through. Don't let it go over-the-top, though, with inside-joke material that no one else will understand or references that will embarrass you in front of your guests.
Pro: Neither of you has to be the world's most awesome writer.
Keep your vows short and sweet and from the heart, and they'll be perfect.