Hello all, I've come to this a little late but it's interesting, as I'm sure many aikido practitioners ask questions along their own path.
I have 14 years under my belt (so to speak) but recently saw some boxers in training and was taken aback by the speed and power of the multiple punches.
A friend who trains with me used to box so he brought his gloves (for safety) and we 'messed about'.
There are many valid comments in this thread, some I understand, some I think missed the point, BUT what I found in our 'messing about' was that you have two options - 1) stay out of range and look for an opportunity or 2) committ totally without any form of hesitancy.
I am sure our few sessions did not take us far but I see either committed frontal irimi or irminage (getting behind) as most effective.
I did think that gedan attack (take the knees out with your whole body) would be so very unexpected.
And yes - many factors such as awareness (why are you there inthe first place); what are the circumstances of the situation etc. are SO relevant but I look at this as purely another question as to what do we (I) know. (know = where am I along the path?)