Positives first: they are really nice - and go along some scenic areas of the river (as much as you can have a scenic muddy swampy polluted river slowly running through an urban area). They can be quiet during the week, although I am sure they pick up on nice weather weekends. And their quality is actually quite good as of 2011, no major asphalt problems or too much debris (although there can be some after storms). They have a fairly decent length for casual rides (not long enough for anyone wanting a serious ride).
Now my complaints... They are very very hard to get to without a car, especially for children or the all important "Interested but Concerned" category of rider. The Memphis section is connected via a very nice bridge (pictured below) to Shelby park, but Shelby park is also pretty hard to get to without a car as well. They are almost completely inaccessible from the north unless you want to go really far out of your way. They can be reached from the south however the area is massively suburban and car oriented - which means in places you have to ride on huge fast moving multi-lane major roadways with no sidewalks or bike lanes - and there is almost no way to cut through slower more calm neighborhood streets because they are full of streets that go nowhere but cul-de-sacs. In order to get there by bicycle you either have to put yourself in mortal peril or take a 2 mile round-about-detour and both of those things sound very bad to my wife and kids.
Or you have to drive there.
Since they are completely in the suburbs, the greenways don't really go anywhere. I know that sounds silly, but when they don't have destinations along the way - the greenway itself is the destination, and while that is nice - it gets boring. The Germantown section links to lots of small office complexes, a lot of professional buildings, near the east end there are a couple restaurants nearby (I think a Waffle House and a Perkins) - and a few gas stations. If you venture down the really big loud busy Germantown Road across the river there is a donut shop, another gas station, and a Lowes. Farther south on busy Germantown there is quite a bit but not easily bikeable to. On the Memphis side there is really, uhmm - nothing. There is a gas station and the Baptist hospital and all its associated professional offices. Sure, there are stuff in these places - but there needs to be places that people will want to ride to, to recreate at, and to return to on a regular basis. For a nice bicycle or pedestrian route, you want there to be a reason for people to use the path regularly, over and over - for the path to become integrated into the weave of the community. These paths are so sterile and the destinations so not bike+ped destinations that the whole thing really only exists for its own benefit. (Note, it is possible to get to the stuff at Germantown/Exeter and Poplar riding south on Brierbrook - but it's not close nor immediately obvious from the greenway).
Of course the Memphis side of the Wolf River Greenway does connect to Shelby Farms park - which does have a lot of stuff going for it, and through the park you can get to the Shelby Farms Greenline which does head into midtown - and midtown has a LOT of desirable stuff to do and places to go. But Shelby Farms is a HUGE park, and Midtown (or Downtown beyond) are pretty far away for all but fairly serious riders.
The gaps in the Wolf River Greenway and the serious lack of neighborhood connectivity to it present real barriers to their use for both bicycle commuting and for family recreation from the nearby neighborhoods. The lack of along-the-way destinations to bring people and bring them back reduce the recreational value of the greenway.
As it stands currently, the Wolf River Greenway is a very nice area with a somewhat limited value in my opinion. Unless you are a serious nature enthusiast who will go to the area regularly, once you have ridden the greenways - there is not a ton of reasons to do it again. Sure, some fitness types might incorporate the greenway into their workout routines as part of the route or loop - but they really need to start working on making these things really usable by the community at large.
What can they do?
First, fill in the gaps. The greenway should connect all the way from Germantown road to Humphreys road easy enough. We can argue that the ultimate goal of going from the Mississippi river to Colierville is damn ambitious - and that would be seriously cool - but I would like to see just this one section which already mostly exists become useful first.
Second, give people a way to get there without a car. Put some wide protected bike lanes on Kirby between Poplar and Wolf River Rd - Kirby has tons of room in that area (and as a bonus it would increase access to the businesses at Kirby and Poplar). There should be good pavement and crosswalks and maybe even a light for bike+ped crossings across Wolf River Rd. at the Poplar Estates parkway (north terminus of Poplar Estates Rd. which currently has a path worn in the weeds up to Wolf River, and requires people on street or road bikes to get off and lift their bikes over curbs and across a ditch). Neshoba between Miller Farms and Riverdale needs a bike lane or at least a shoulder or sidewalk. With better access - improve signage and wayfinding (not too bad on the Shelby Farms side of the river).
Third, work on getting some destination entertainment businesses to locate in the Baptist hospital complex, and down around Wolf River and Germantown roads. Things like family friendly sports bars, so people could ride their bicycles down to watch the game on the weekend. Perhaps ice cream shops, book stores, and other things which people could enjoy on a break in the middle of a ride. On a warm day it is nice to ride for a while, break for lunch and enjoy some iced tea, then ride back home. Put a pub or two to get the twenty-something and the hipster crowds who are riding bikes everywhere.
Finally, this may be a stretch for Memphis, but make sure that transit lines go there with bike racks - and (gasp!) run those lines on the weekends. Someone who doesn't want to ride 30 miles round trip could use the 34 line to connect from downtown or midtown to Shelby Farms and the various greenways in the area - if the 34 ran on the weekends. Transit helps - it can be used to cut the total length of a ride into a manageable size, or to come home if you get a flat tire, or simply to meet friends.