I don't use it. I may have a lost a job or two because of it, but I'm fully TelePrompter proficient, and they normally have cue cards up at auditions. The only time I get into trouble is if they want me to do it without my glasses. Even then, though, I've usually studied the script enough to get by on memorization.
But some people are more comfortable with the ear prompter and are really good at using it. But I think it only really works in a solo situation. When you're playing host/narrator and you have a bunch of lines and you're the only one speaking.
Twice now, though, I've been in situations where I'm paired with someone and they're trying to use ear prompter and I'm not. And both times it hasn't worked at all.
The timing gets all gummed up. Especially like today. I took a half-hour last night and some time on the bus to memorize the lines. Not verbatim, but enough to paraphrase and at least get the cues right for my scene partner. But she was locked into her ear prompter, and if I said something a little differently or it took me a little more or less time to say it, she'd get all messed up. I'd say something and there'd be a long pause. And we were supposed to be a husband and wife having a normal conversation in bed. It was ridiculous.
Also I think sometimes people get totally locked in on a specific read. They've got their lines recorded and are repeating back what they hear in their ear and they're not making any adjustments. The director gave us several specific notes and she didn't really take them at all.
That might be something other than an ear prompter problem, but still. They say it frees them up to focus on the other person and the relationship and stuff, but I think having the lines in your head (generally) is a much more natural way to go.
Also, internalizing the lines helps you better understand what's actually happening in the scene. It was clear she didn't "get it" on one of the key important points, and she never quite got it. This, I think, is where theater training really, really helps.