Obviously it depends on the individuals and their relationship with eachother, but in general if it feels right, mums are great birthing partners.
Just think about it - men didn't used to be allowed anywhere near their wives during childbirth, traditionally women have always been supported during childbirth by close female relatives and friends.
It's certainly not uncomfortable or disrespectful to your partner, as one poster suggested! That's a pretty ridiculous thing to say really. Yes, childbirth is a special thing, but it's also a messy chaotic event and if you're giving birth in hospital you're going to have all sorts of strangers around you at one stage or another - anyone from ambulance drivers, to cleaners, healthcare assistants, nurses, midwives, doctors, anaesthetists, etc! It's not a magical glowing Disney moment shared only between you and your partner. Your own mother has presumably been through childbirth herself so is an entirely appropriate person to have present.
Having both my mother and my husband with me during the birth helped greatly as it meant I always had someone by my side even if one of them needed to pop to the loo or grab something to eat. This is especially useful during long labours. I eventually had an epidural and the labour was progressing slowly but consistently, so the doctors were able to tell me that I wouldn't be giving birth for several more hours as they had a time estimate in mind. Having my mum there with me meant my husband could leave the hospital, get something to eat, and return with plenty of time to spare before the baby was delivered. It was also nice to have two people supporting me, fetching me drinks, applying a cool flannel to my forehead, etc!
I would definitely have my mum with me again but unfortunately she'll probably be busy babysitting my first born next time round! I am considering asking a friend to the next one...! I think it helps DH too, not being the only one there with me.