The GDL is quite accepted amongst barristers and solicitors. Do remember that to qualify there are other courses you have to take after the law degree/GDL and these cost a lot. (Solicitors do the LPC - around £9-10,000 for the course on average, and barristers would do the BPTC - around £15,000 I believe) And those are before you qualify. The GDL itself costs a lot too.
If you're lucky and get a traning contract/pupillage a few years in advance (for most places you apply a year or two in advance), some of the bigger firms/chambers will pay for your GDL and LPC/BPTC, and those are vey competitive to get.
As for the course itself, I did the LLB degree, but I have a friend who is doing the GDL and says it's very hardgoing. You're basically doing a whole degree in one year. I think doing an actual law degree is the better route, but from an employment perspective, firms/chambers generally don't mind.