Managers are a great way to start off in the business. Don't discriminate - especially if you don't have a lot to bring to the table.
Agents are a different breed than they were a decade ago. It is harder to find an agent who will simply take on a client because he believes in the writing. Nowadays, agents expect writers to come to them with scripts they can sell. (That amounts to about 1% of the writers who make contact.) And it helps if a writer has just won the Nicholl Fellowship or has some kind of heat.
Don't have that kind of pedigree? Then work with a manager who will introduce you to an agent when the time is right.
Managers are more open to working with new writers, nurturing and cultivating them - which includes helping to guide the writer toward projects that are more saleable. Yes, that might mean grooming your work or even you.
All of the writers I know who have recently broken through in the spec market did it with a manager first. (In some cases, the manager had the writer make the rounds to agencies and sign with one right before the script was taken out.)
But managers aren't miracle workers. And not all of them are good. And because their clients have IMDB credits doesn't make them credible or effective. (Maybe those credits were earned before signing with the manager.)
What is their perception in the industry? Who are their clients? What have they set-up lately? What have they sold lately?
You should be directing these kinds of questions at them and expressing your expectations of them (and, quite frankly, not wondering out loud on a message board).
Ultimately, representation is only as good as the client. If the writer isn't in tune with the marketplace or sucks in a room or cannot deliver on assignments, there's only so much a rep can do.
I never feel that a writer should sign with the first rep who comes along (which many writers do because they fear the opportunity won't come again) - unless it feels right.
But a manager (who should only get 10% - not 15%) can definitely be a step in the right direction.