I didn't for my first birth and I am for this one because I am attempting a VBAC. Having a doula is supposed to increase my changes of this and every little bit helps! It also takes a bit of pressure off of DH so he can also enjoy the moment.
Not only can a doula offer the emotional support you will need, but she can offer ways to try and counter-act any pain you might feel during labor...using various massage techniques, or keep you focused and keep you relaxed. She can also be an added advocate for you if you are doing a hospital birth...like reminding the nursing staff to not ask about pain levels, or whether you want an epidural, etc.
I did not use one with my daughter in '08, but I have just hired one for my next delivery this Christmas. The one I have is trained in Hypnobirth as well as Reiki massage, etc. Really, a doula can't hurt! Good luck!
Pick up a copy of "Birthing from Within" from the library and read section on doulas. They reference a great study that compared interventions, labor length, c-section rates for non-doula births an births with doulas.
This is my first pregnancy but we are excited to have a doula. We are paying quite a bit for her ($1000) and get a few pre and post visits as well as the labor assistance. She has also loaned us books and DVDs. I'm excited to have the guidance as I want to labor at home as long as possible and also to have it be a positive experience for DH and I.