It can ruin your finances and basically leave you with peanuts to spend,so be careful when you do it.
Like how njengamn says,don't be an idiot and make it a ridiculous £50million on a 48-month period.That means you would lose most of your transfer budget for the season (possibly more) even when you're not even managing them!
I've done it in my Arsenal save where I'm signing Jesus Navas from Sevilla for £9.25million cash upfront but £20million being paid over the next 48 months...I am not sure how this is going to go down with my financial status and with what the board thinks..
Generally a bad idea, as they'll also require more money in total. Unless you're VERY sure that your club's economy's gonna get better through Europe, cups or league (maybe even just play lots of games against high-profile teams) you're better off not doing it.