I have a 6 week old boy (my first baby) & we had a doula at the birth, as I was really terrified of the pain involved. So I wanted to add my experience.
I think it's definitely important to choose the right person... We saw a couple of doulas (& emailed with some more) before we chose the doula we worked with. For me it was important that the doula we chose was not anti-epidural, or at least that she could support & accept my choice to use an epidural. I felt with one doula I talked to that she was very anti-epidural (& could barely hide this). So in the end, we chose someone with whom we 'clicked' & who openly discussed with us the pros & cons of epidurals, but clearly accepted & supported our choice & preference, to hopefully have a good birth despite the problems that an epidural might cause. So my advice when choosing a doula is to make sure she's on board with your birth choices, as I think lots of them are vehemently pro-natural/no pain relief-birth (or at least that's the impression I got, I may be wrong!)
In the end, my labour was very hard- or at least I found it very hard. 15 hours natural / no pain relief labour at home (with intense 3-mins-apart contractions) with very slow progression (we went to hospital & got sent back). Then an epidural, which was a blessed relief. And then unfortunately a forceps delivery, because of my panic at the last minute when the epidural wore off & I suddenly felt the pushing sensation. All in all, the doula was most helpful at home. She was calm, had ideas about spending the time, helped me & my partner to cope with a labour that I was finding very hard & that was far too long for my own endurance levels. At the hospital phase of my labour, our doula was again helpful, but when I was given the epidural, I really didn't need that much help, I mostly rested. Then at the end, when I 'lost it' & my labour turned into an ER-episode (rushed to theatre & had forceps) I felt, inexplicably, embarrassed & ashamed that my doula had witnessed that. So much so that this plagued me afterwards, & I kept thinking of my labour & crying. I didn't feel embarrassed & ashamed about my partner seeing me panic like that & ask for help from doctors... So I'm inclined to think that either it had to do with the fact that essentially she was a stranger (although we had gotten to know each other well by that time) or alternatively that it had to do with the fact that she had a very positive, and I would say even a bit idealized, view of what labour can be.
If I have one complaint, it was that I sometimes felt (and this feeling I still have) that she could not accept afterwards or during the labour, that to me the labour & delivery was not a good experience,but one of the most difficult days of my life, which I took a while to digest (& I'm still upset to remember). She insisted that we could see the good parts in it... but for me the good part was having my healthy, beautiful boy at the end.
All in all, I would definitely recommend having a doula, she provided much needed support for us when labouring at home. But think carefully about how comfortable you will feel if things don't go smoothly, and she ends up seeing you in extreme pain/fear/panic etc. Will you be ok with her seeing that? In my case, I found it hard remembering myself in those circumstances, let alone accepting that someone else, a relative outsider, had seen me like that too...
Hope this was not too confusing!