I was recently in Florida and I had a chance to ride half of the Pinellas Trail in Pinellas County. It's a great ride, and worthy of its place in the rails to trails hall of fame, but there are a few missed opportunities and issues that keep it from being a great transportation facility.
From a recreation standpoint it's really nice. It could use a little more shade, but it's a pleasant, flat, mostly uninterrupted ride. There are several grade-separated crossings at major roads which really helps, and there is a lot to see along the trail. It evens passes by a KOA site. The highlight of my ride was the causeway bridge over Boca Ciega Bay.
The 38 mile long trail still isn't quite finished. They plan to add another 9 miles in the next few years, plus tie it in to some other trails to create a loop and also use it as part of a trail across Florida. A lazy person's coast-to-coast trail. But the incompleteness is not what prevents it from being a transportation facility the way the Capital Crescent Trail is.
One big problems is that there just isn't much to tie the trail into. While there are many bike lanes in the area, most of the ones I saw were along what some would call "stroads" - wide multi-lane roads with higher-speed traffic; and even with bike lanes, these roads are not going to appeal to the interested but concerned.
Where the trail did come near more bike-friendly residential roads, there was often no way to get to them. Several times I could see such a road just a few feet from the trail, separated by a ditch, but no connection was made to that road. Sometimes, there was even a fence in between.
The other issue was the lack of signage. While there was usually a sign telling a trail-user what the roads ahead was - especially useful for the many overpasses - I didn't much telling me where that road would take me. Such information is important for getting regular trail users to think about using it to get to those locations, I feel.