@vickster has it right
For a regular commute you would be wasting money. After a few rides you will have memorised the route. If that's all you're going to do, do the following:
1. Go on the website
www.ridewithgps.com
2. Have a go at planning your commute
3. Use the google maps tool on the planner (drag the little man icon) to any junctions etc you don't know. This way you can have a look at them and familiarise yourself
4. Ride it with the route on your smartphone. Don't use the smartphone to navigate, just use it in the event you forget the route
5. Refine it
Of course if you plan to do other rides on roads you don't particularly know then it may be worth investing in a GPS device. But for a regular commute? Definitely not