I think there are two types of people when it comes to seiza...those who can and those who cant! I am one of the latter!
I know people who can sit in seiza quite comfortably for ages basically because the make up of their legs/knees allows them too...these are the people who have sat like that since they were children so its no biggy.
Then you have people like me who just cant sit in seiza with any amount of comfort! Its not because im inflexible...im pretty bendy....but because the backs of my thighs along with my calfs are too big so when legs are bent in seiza i just cant sit down properly!
Along with that and suwari waza technique i have suffered a knee injury within the last 6 months and its no fun......torn miniscus apparently...its not nice....very frustrating too! This happened on a weekend seminar when we did a session of suwari waza (about 1 and a half hours). It wasnt painful straight away but over the next week i couldnt walk up of down stairs!
So anyone out there who does a lot of kneeling technique....take it easy and in small doses, there is no point in injuring your knees now to cause greater poblems later in life.
Knees......you only have two of them and for aikido you need both!