i have been using whmcs for years and it is the best software in my daily use list of tools. it is superb and i can not talk more highly of it.
primarily it lets you manage your domains and hosting clients and automate every aspect of it, but it also lets you issue quotes and convert to invoices among many support related features which just work brilliantly.
the project management tool is new and has potential, but its not as slick as the main system yet. i bought it as i was so happy with whmcs generally, but its over priced and not bug free but saying that, its a one off fee and upgrades will come free in the future, so hopefully as more people take it up, more time will be spent on it.
yes, i think it will help any web designer to deal with their clients effectively and in a professional manor. my advise is to try it