I'm going this year for the first time, staying in the steinberger parkhotel which is supposedly convenient - for the hightime roaming around old town at least.
Yes, dusseldorf is reputedly very good, and becoming (or is already?) the single most important boat show in europe.
I've been there a couple of times and its simply brilliant. Much much larger than the London Show, Earls Court at least, and loads of boats you will never have seen before. Organised with typical Germa efficiency but the first time we went it was at a weekend and I would definitely reccomend a weekday visit. You will need 2 days if you really want to see everything
I cant remember where we stayed but it was near the Altstadt area which is worth a visit. I dont recall any hotel booking problems