The thing with grammar schools is that they do more or less guarantee that your DC will take and pass (often with top grades, though not always) GCSEs in academic subjects only. No childcare/tourism/media studies options.
So it depends on what you want and the type of child you have. A lazy but clever child may choose the easier subjects on offer at a comp which are then not acceptable at university later on (even though it is suggested that all GCSE/A level subjects are equal, unfortunately the universities do not seem to agree....). I know one of my DCs would have done that, and I would have been under intense pressure to give in. At least he achieved a decent set of GCSEs and A levels which enabled him to apply successfully to top universities.
Obviously comprehensives also offer a highly academic education too, but I imagine it could be harder to go for that option when there are other less strenuous options on offer!
I would say, though, some of the teaching at a grammar school leaves a lot to be desired, many with the attitudes of 'oh he'll get an A in X subject, they all do' (he didn't :)).
However, as you say, you have to be sure that they will pass the test in the first place which cannot be guaranteed in any way (although some grammar schools are less selective than others I believe) so it is a bit of a risky business.
It is also easy to think that they will naturally work hard, buckle down, choose sensible options when they are farily biddable primary school children. It's a different kettle of fish when they are teens which is why I preferred them not to have the choice really. Personally, I wanted mine to have a good grounding in academic subjects just so they would have a proper choice later on in what they wanted to do and not be held back from anything because they hadn't got the 'right' qualifications.
Another thing about grammar schools (which for my 3rd DC was the more important than the academic side of things) is that they tend to be smaller and have less (possibly no) bullying issues. It is ok to be 'clever' and no picking on for being a nerd or whatever. The other side of that is some children who have always been top of the class at their primary, won't be at a grammar school. They are all much of a muchness and some children don't like not being top dog any more!!!