I actually did this for my last cider, and the results were interesting. It was quite good, although it did have some drawbacks. Overall I prefered the syrup version than the plain.
Basically the finished product ended up being Darker, Sweeter, More 'appley', less clean and crisp, and Fuller Bodied than my previous batch without the syrup. Final gravity was 1.005, the highest ever from any cider I've made!
It also ended up with a touch of stewed-apple flavour, but it wasn't hugely pronounced. Whilst it probably is strictly an undesirable aspect, it wasn't offputting, and most non-cider-drinking people that tried it didn't notice until it was pointed out to them.