Really normal. I find most women really love this. Some orgasm from it.
The key question is whether you like how it feels. Women's nipples vary widely in sensitivity.
For example, some mothers orgasm while nursing -- some really have big ones, and others have mild ones. Other women are hardly sensitive at all there.
I'd say maybe 50% of lovers do this. I like to. It is quite arousing. But I only do it if my partner enjoys it, or at least doesn't have a problem with it. I find it very fun, to say the least.
As long as he doesn't go, 'Bitty, bitty!' before he starts sucking, I think you are probably fine. Sucking gently on a womans breasts and nipples is very sensual and sexy, it's the feeling of that warm flesh between your lips that feels so good.