Your concerns - or rather, that of your friends, are common and well-founded. Aikido, when performed right, is so effortless and fluid it looks like uke (or Tori) must be helping things along. That's not the case, however, as you're learning by now. Aikido is effortless because it makes effective use of physics, geometry and anatomy to defeat an attacker, not brute strength and speed.
This might help a bit: During class last night, I wound up helping a newer student while we were practicing zempo-nage tenkan; if you don't know it, it's a devilish little technique that looks like you're just turning and waving one hand up and down a bit - but sends uke flying Superman-style across the mat. It's so nasty because it's incredibly effortless, totally unexpected and potentially very dangerous - the attacker could easily land on and break his neck. Which is why, of course, uke rolls.
(The new student was rolling before our hands even touched - very common to new ukes; he was missing the point a bit.) You can tell your friends what I told him, "This isn't a team sport; the only reason uke rolls is to save his own neck."
I agree with Kelly too; don't worry about your friends, it doesn't matter if they believe it works or not. If they don't believe it, invite them out to the dojo to see it done real-time; who knows, they might be convinced and sign up.
(Of course, you could be a real jerk like me; learn nikkyo and then go back to your doubting friends, hold out your hand and say 'shake!' heh heh heh.)
As for my own experience in Aikido, I'm a relative newcomer myself, I've not had the need to use it 'on the street' yet, barring a situation a few months ago I was able to diffuse, but I've been a soldier of the Canadian Armed Forces for most of my life; I'm a skilled fighter and know what works. Believe me; Aikido works, even if mine doesn't work very well - yet.
I hope that helps to answer your questions; and that of your friends.