I've been through it all with the WR. I've had the bugger off letter and I've also made it through to the highest echelons of power (

). It is a bit of a PR exercise but there's success to be had if you're persistent.
First thing to note is, as Stijn Hommes said upthread, they're not looking for scripts or ideas. They're looking for writers they might *might* want to work with in future. It's entirely possible that a script could be read, bought and produced but this is in the vast, vast minority. I've certainly never heard of it happening.
A script I wrote went through the system which did very well and got me meetings with the head of the writersroom and the head of writersroom in Manchester as well as the head of continuing drama at the BBC itself. Both were sort of general introduction/chatty meetings but the idea is that you go in, say what you want to do (you need to be specific with this, as in, naming the shows you like/dislike, you career ambitions etc) and, in theory, they do their best to help you make it happen. They're not going to put you straight to work on any show no matter how good your writing is though and in the best case scenario, they'll put you on one of their shadow schemes or get you a trial for one of the existing shows. Be warned; this process is excruciatingly slow. My agent started prodding them for a trial on their most new writer-friendly show, the one most people start out on, and it took 12 months to get it.
The whole thing is certainly no better or worse than going through other production companies. They will read absolutely anything you send them but you are at the mercy of the readers who sit in an office and read scripts all day long. When I went in there was one wall just stacked with padded envelopes that hadn't even been opened yet.
So if you're looking for a way to get your writing onto existing shows at the BBC then it's a good route to go assuming you've got the work to back it all up. But if it's a case of submitting something original you're hoping will get made, it's a severe long shot. The only thing they'd probably be more willing to take a chance on with an unknown would be a sitcom because the BBC are desperate to make good sitcoms again.
Hope it helps, good luck.