We don't seem to have a separate Yelp entry for the north shore trail and the Strip District to Point State Park trail so I hope it's ok that I combine reviews.
First the Strip District portion would get 5 stars. The vast majority, other than a small bit by the Cork Factory, is paved. Maintenance is kept up. The views are just wonderful; I love going underneath the bridges and coming out by Point State Park. This is part of my biking work commute to downtown from Lawrenceville and I greatly enjoy it. The hills are pretty gentle unless you're vastly out of shape and slow like moi. I also take my senior citizen pup for walks on this trail and he enjoys himself so much that he can't help but nap all day after the long walk. A tired puppy means a happy puppy and master. I do wish they'd properly connect Lawrencevillle with the Strip though.
Secondly, the North Shore trail. This section has less ups & downs, it's mostly flatland. It's also very pretty and affords you a nice look of the stadiums, casino, science center and the confluence. As others have noted, it can be a bit muddy but I bike this area so I'm ok doing some puddle dodging. There's a connector with Herrs Island which is a nice residential community to pass through. Who knows if they like all those bikers and runners though.
This review would be 5 stars if not for the sometimes muddy nature of the North Shore trail or the fact that I can be easily slowed down by the winds along the rivers. But still, I'm a huge fan of both trails.