Do it. Ours looked like new - we honestly hadn't realised the window into the oven was clear glass till he'd finished. Lasts a whole year. Best £60 I spent all year. Well not really - but you get my drift....
We do ours every year before christmas (book now, they get booked up!) for about £45-50. Totally worth it. Then once or twice a year I bung one of those Oven Prides in and give it a quick rinse and that's it.
I would have thought it was money into thin air before, but we had to get one when we moved into our new house as the oven was in such a sorry state and now I'm a convert!
Had mine done last week, after a year of owning the oven. Wasn't really bad but needed a clean especially inside. And I didn't want to do it - its a big job and a dirty job.
We have a 3 door range type electric oven, with 6 shelves in total, and on top there are 5 gas hob rings, and a larger extractor above. They cleaned it inside and out, removing the 3 plates of glass in each door, changed bulbs and filters, etc.