The road is good and it's a beautiful route down to Napier. I found roads in NZ much more well sign posted than in the UK and of course, you're not likely to hit much traffic
You're not likely to get lost. Just be sensible and allow plenty of time for your journey. If you drive you can stop whenever you like as opposed to being on a bus when you have to fit in with an itinerary. I know a few people that have travelled by bus and regretted it. If you drive though, you might feel a little more tired on arrival. Don't know what your plans are but could you arrive arrive a day or so early? That'd give you a chance to have a little look around the place too.
We loved Napier. We stopped there for a few days on our NZ road trip. If there had been work there for us we would have been tempted to stay. We thought it had a lovely feel. What kind of work do you do?