For the most part, they're hassle-free and decent spots. Some gotchas:
A few of the hiker-biker sites are pretty gross-- in dirt fields right next to a restroom and under bright streetlights. Unfortunately, I can't remember which ones are bad, but if you end up at one, it's worth asking if you can move to a real site or set up in a day-use area once it gets late.
But, I'd be careful about asking other campers directly if you can piggyback on their spot. If you inadvertently come off as sketchy, or the person is uptight, they can complain to the ranger and create a headache for you.
As other posters noted, plan your itinerary so you stay at the parks listed with the "HB" icon. In my experience, if the park doesn't have a HB site and it's full, they can and will turn you away. At that point you need to be a smooth talker or a strong pedaler.