I'm going to disagree with a lot of people in this thread. A lot of aikido techniques, even when done properly, do inflict pain on the uke. That is not to say that goal of the nage ought to be to inflict pain, but I would think there is something wrong if I spent half-an-hour practicing sankyo and felt no pain at the end of it.
I am still very much a novice, so you can take my words with a grain of salt if you like, but I think pain is an essential element of the martial arts. The unique spiritual benefits offered to us by the martial arts are rooted in the opportunities they give us to face our own fear, insecurity, and pain. If we discard these opportunities, we might as well be practicing yoga.
Well, after 28 years of teaching Aikido and experiencing Many different styles, from Ki Society to working with Seagal and just about everything in between, I continue to practice and teach painless Aikido - but that being said, I have always said to each his own.
Aikido is much more than "just" a martial art, with unique and deep qualities behind the experience of both practicing it and "receiving it." To me, facing one's own fear etc. aren't necessarily spiritual benefits, but personal ones. The spiritual benefits and experiences, to me, are far beyond that level.
And yes, I agree, Aikido -practice- often isn't painless - because people tend to do it wrong for a while....