I find ukemi to be the biggest stumbling block (excuse the pun) in aikido. Beginners (esp. older ones and females) often hate rolling and many people who start aikido for self-defence wonder why they need to practise ukemi at all.
We do ukemi practice at the start of every lesson, but usually I make sure that it is also related to a technique (e.g. kokyu-nage, kaiten-nage); the techniques being done very gently.
For beginners I do some special additional sessions so that they rapidly learn to ukemi(I find people who are slow to learn ukemi loose interest quickly).