As far as I'm informed (second hand knowledge, so no guarantees here), the Wanderlust maps are not routable, meaning that you can not just have your GPS unit guide you to a town.
They might have a different advantage compared to the preinstalled basemap in your GPS though. For many parts of the world (Africa, most of Asia, ...) the basemap is just a featureless plain with maybe a couple of the most important cities and highways on it.
The Wanderlust maps are supposed to be much more detailed than that, so you will be able to see where you are, where the streets are going and where the next town is.
What they - in essence - give you is the same that any decent paper map will do for you, with the most notable difference that your GPS puts an arrow on your exact position.
Should you take them? Well, that depends on...
- ...where you are going, Garmin Worldmap might be good enough or there might even routable maps available.
- ...how far off the beaten track you're planing to venture. Most major highways and throughfares are signposted almost round the world. Where they are not there's possible someone around to ask. The further off the beaten path you go, the more important it might become to navigate on your self and not get lost too much.
- ...how much you enjoy playing with shiny new toys. For some reason I just simply enjoy using a GPS more then the ordinary paper map. You might be different, though.
- ...how well you have planned your trip on paper maps. If you know your maps, have done well so far with them and will refer to them whenever slightly in doubt, I can't see much use in buying electronic maps you won't use much anyway.
- ...of what quality the Wanderlust data is in comparison with the data of other maps. If there's a significant difference you obviously should go with the better maps. However this is a bit of a blind guess, since you probably won't be able to have a look at them, without buying the whole package.
- ... how much you want to support the guy who does the Wanderlust maps. As far as I'm aware, he is a true overlander who supports his travels and work with the sale of his maps. That may or may not be something you want t support. Whatever floats your boat there.
So, I'd say there is not definite ya or nay here, it all depends on a lot of factors and - as with any piece of kit - it might be incredibly useful or just a waste of money, depending on how, when and if you use it...
Have fun on your trip!
P.S.: I thought about buying them, had no money at that point, went without them and was fine in the end, but then again that's just me and I never left Europe on that trip.
P.P.S.: This is the Garmin basemap, off the beaten path: