Hi - my DS was at InterHigh for yr 10 and 11. There's a couple of other mums on here with DC in Yr 7 and Yr 8 now who I expect will pop along when they see your thread title and give you their impressions too.
We certainly found it worked well for DS, and he got about the set of qualifications through it that we'd have been expecting him to get in ordinary school.
The pros and cons you can probably guess yourself to a fair extent - you've got qualified subject teachers for each subject, and your DC is learning in a group which gives them a "peer group" and people to bounce ideas off, work with etc. You can also be reassured that they're getting the range and level of subjects they'd get in school, for the academic side.
OTOH it is only the academic subjects - no music, art, PE, cooking, etc, etc... The class will be going at one speed, there's liittle scope for "setting" in subjects where your DC is going faster or slower than the average for the class.
You don't get any flexibility - you are tied to a timetable for the day/week/term/year, and to a set of subjects which might or might not be what your DC would have chosen. So it doesn't have the advantages of flexibility and tailoring that HE can give you.
Whether that seems like a good plan for your situation just depends really - for us it was a practical solution as the school way of learning suited DS fine, and the timetable left plenty of time for music which is what he wanted to do and couldn't fit in well with the other schools available to him.
But it depends what your reasons are for thinking about HE, whether you think she'll go back to an ordinary school, how much you/she like flexibility etc...