It depends on what you need and how you like to birth (if you know). I had a doula with my first birth. I was having a home birth and having never given birth before I didn't quite know what to expect and having a doula is just the done thing. I honestly felt like my doula was mostly in the way. I preferred to be alone anyway and was quite comfortable just getting on with it. I didn't need much support and actually for the bit of time when I did need support and my husband was providing it, I'm truly not sure where she was (like I really don't remember, but it wasn't with me, which was probably for the best). I realised after the fact that I don't like many people around me giving birth and being in my own home anyway, I felt in control, relaxed, didn't feel like I needed much from anyone other than to just stay out of my way. After that, I said I wouldn't hire another for my 2nd birth.
Well, I'm 36 weeks now with baby #2 and having another home birth, but I have actually decided to hire a doula this time. Again, I don't personally feel like I need much support and my doula knows this, but she is there 'just in case.' Her main role honestly is to take over at home in the event we need to transfer into hospital. I don't expect I will and I had an easy, straightforward birth the first time and this pregnancy has been even smoother, but you never know. Our older dc will be with us for the birth and we don't have friends or family who could come quickly in the event of an emergency (will definitely take at least 2 hours, meaning otherwise my dh would have to stay behind and I'd have to go into hospital and birth alone). So our doula is mainly there to take over at home until family can arrive relieve her. She's cool with this and knows I may not need much support from her and is happy to just sit somewhere out of the way with a book until I need her. She'll mostly provide postnatal and breastfeeding support. So for me, it's definitely worth the peace of mind knowing she's there just in case, even though I don't expect I'll need much from her. It means we don't have to worry about my husband having to leave me in hospital by myself due to childcare issues. That means I can be relaxed and just get on with it.
So for me, this time, definitely worth it because I know exactly what my needs are. I was less sure really what I needed the first time so my doula wasn't as helpful and was kind of in the way. I think it depends what exactly you need support for, if you want extra support at all, how involved your partner will be, etc. First time around my dh did a lot of the things a doula might have done (filled up the pool, physically supported me in labour, etc.), but now that we have an older dc to look after, he won't be able to be quite as hands on. That said, they don't have to be ridiculously expensive. Both times I've hired a trainee doula. The first time (though she a little bit sucked) the fee was £200 and this time £400, which includes all antenatal and postnatal appts as well as the on call period and birth.