ABSOLUTELY! A good guide can give you a great base for orienting yourself for the following days - many guides tell me that their guests actually book another tour because they find that the time spent with them was well worth the cost.
If you are concerned about getting the "lay of the land", I strongly encourage you to take advantage of tours and guides.
I'm tourguide and I advice to my tourist to take 4 or even 8 hours tour for the first time (depends on how much would you like to walk). You can easily spend 4 hours walking around city centre and listening the history of Florence and Renaissance. In 8 hours you can visit some museum or even climb the Dome.