have any success afterwards?

I'm curious as I don't generally watch these shows, it's not usually my kind of music. As well as the T4 ones there seem to be a thousand of these contests run by various organisations nowadays. Has any successful band ever emerged from one?

You answered your own question - there are so many of these competitions these days that it is not any guarantee of success.

What it does do is give bands some profile - and in such a competitive scene this is worthwhile.

What bands should avoid is Battle of the Bands run for profit which basically just give out prizes of gear or studio time or even beer at the end. These competitions are designed to earn as much money as possible for the organisers. A sort of "raffle" for musicians - and usually the voting is based on size of audience ( and therefore beer consumption at the bar )

It can be counter productive for a band if they get seen and slagged off on a TV show - especially if that is early in their career.

My advice would be to enter a show under a different name. That way if you dont win then no one will ever know - and if you do win then you can change your name to cash in !!!