There are 2 parks, Islands of Adventure which is where Harry Potter is, and Universal Studios. Both are good parks in their own right.
I don't know that there is a typical universal trip. We've done it twice. In 2004 we did one day at IOA only and we were staying at an offsite house. It worked out great but that was pre-harry Potter. In 2010 we had 4 nights, 5 days but only did 3 days in the parks. We did non hopper tickets so it was 2 days at IOA and 1 day at Universal. In both cases though, it was so incredibly hot (August) we didn't put in full days for the 1st 2 park days.
Do I think it's worth it for a day? Depends on 2 things.
1. How big of a Harry Potter fan are they? If they are big fans...YES it is totally worth it. We loved loved loved loved it. Then again, we are pretty huge fans!
2. Are they thrill ride junkies? Both IOA and US have some of the best coasters, bar none. At IOA you've got the Hulk, TriWizard Challenge and Spiderman, some of the best rides ever plus of course the Forbidden Journey. Several wet rides that are a ton of fun as well.
US has RipRideRockit. We actually didn't make it on that one but there is a ton of other good things.
For me, one day, it would be IOA hands down and you can definitely put together a great touring plan for that.
If you need an afternoon break, the Lone Palm Airport in Margaritaville (which is where the Hemisphere Dancer...a giant seaplane now lives) has a great happy hour margarita 2 for 1 deal. It's a short walk from either park.
I like apples and I like oranges. It's ok to go to the Dark Side and it doesn't take a thing away from Disney to me. Totally different animals.