Totally non-scientific and perhaps b***s, but in my experience children who have been trained to sleep well as infants (and so are "easy" for the parents in that respect) can often be quite "difficult" in other ways - e.g. whiny, tantrummy, grumpy, wild. I don't know if there is a connection.
I would say my two have been averagely OK sleepers. My eldest was sleeping through by around two years, my second is 2.5 now and generally speaking sleeps quite well with probably one rotten night per week. It is true that five years (cumulatively) of disturbed sleep is hard on you and your partner, GF is right about that. DH and I are shattered (and while both mine woke in the night it certainly wasn't/isn't five times a night like some people have to deal with).
On the other hand, while my two of course had their challenging times and ages, they were and are not particularly difficult in areas other than sleeping. They don't hit or bite or shove other children. They rarely grabbed other children's toys at toddler groups etc (though they did occasionally). They don't have 30 minute screaming tantrums. They have both been quite "safe" children even as small toddlers - no extensive baby proofing needed or visits to A&E or running out into the road. I have often been able to reason with them both from the age of about 2.5 and head off a confrontation. They are broadly speaking quite obedient and will say sorry. They eat quite well, considering their ages.
As I say it could be b***s, but I like to think all the above is the payoff for being responsive to them for five years at night!
Of course if you are getting so little sleep that you are on the verge of a nervous breakdown then clearly something has to be done. But I think if you can manage to not force sleep training on them, then that's the best option in the first instance.
(My nearly five year old sleeps like a dream now.)