I discovered the NPRT this summer and have taken my bike on every business or pleasure trip since. It is well-maintained, wide enough for walkers and bicyclists to pass, and gives some perspectives (particularly in the northern half) of the river that aren't easily had from a vehicle.
One recommendation: having spent my early years in Niagara Falls, and only recently returned for family and business matters, I find that tourists are still dangerous drivers to mix with bikes. Thus, I'd recommend that you cycle the on-road portion (from the Whirlpool to Dufferin Islands) as early in the day as possible, to avoid the probability of a near- or actual-collision with a rubbernecker, sudden u-turn or stop, or other unpredictable behavior. If you can't, then consider driving to Dufferin Islands and using it as your base to go to Ft Erie and back, and park at the Whirlpool Aero Car (there's an upper part to the lot) and use it as your base for the leg to/from Niagara on the Lake.
And finally, because the trail follows an old street-car line from the Whirlpool to Queenston, there is a gentle grade, off road, when one descends/ascends the escarpment. It makes the only really daunting part of the trail (other than the on-road portion) a pleasure.
If you own or manage Niagara Parks Garden Trail, register now for free tools to enhance your listing, attract new reviews, and respond to reviewers.