I find that they're good in general. It's nice having stuff fit more loosely. Also, when you're getting bigger and bending at the waist becomes more of a nuisance, they help you avoid putting pressure on your lower abdomen.

I wouldn't feel compelled to rush to maternity clothes, btw. Buying a few things one size up (especially tops, which can be looser than pants) has gotten me through almost my entire pregnancy.