Been wearing one for just about 2 months now. I was pretty hesitant about the whole idea in the beginning ,thinking it would hinder my freedom of movement... yeah it certainly does at times. It has forced me to pay a lot more attention to where my feet are. It's hard to stand up when your foot is on your hakima. :P lol
I don't know that I'd say it has been hugely helpful in training itself but for me when I began to wear one I felt different. It has, for me a feeling of formality and finish. More polished, more serious more a part of the tradition and history of aikido.
I like the feel of the belts and ties and even the koshita. It seems to give me some great lower back support and makes me stand up straighter. It makes me more aware of my center by the fact that I feel all of that heaviness of the hakima hanging from that point. It gives me more awareness of my hips and reminds me to use them. And yes it does make swari waza and seiza a little easier on my knees.
I would not say that wearing a hakima has made my training better in any specific ways but it has enhanced it in many small ways.
And besides all that it looks really cool.