More and more Lipo's now say you can charge them at more than 1C (I have 1 which says 6C!), what impact does charging at over 1c have, is it dangerous? Shortens life a little/lot? Any other effect!?

Also, should it be at a specific rate or doesn't it matter, eg if I have a 5000mah pack which says max charge is 2C does it matter whether I charge at 6/8/10A etc?

No effect other than to shorten charging time. Provided you have a charger that is capable, hugh C rate charging provides a shorter charge time. At 6C you would be charged in about 10min. There has been extensive testing by these lipo companies and there is no apparent loss in cycle life.

The advised C rate is a maximum. Using lowered C charge rates does do any harm. It doesn't do any 'good' either as far as improving cycle life.

I know the manufacturers say they have done extensive testing showing no loss of cycles with higher than 1C charge rates, but I dont believe it for a second. I think its more of an "acceptable" loss of cycles. This is just my opinion and not a fact.

Charging at 3,4,5,6C is really not worth even debating because 99% people will not have a charger capable of these charge rates (>10amps).

I've been charging lipos at 2C for the past 3 years and not had a single issue with either safety or loss of cycles.

Sure, charging at 4-6C could more than likely results in 10-15% loss of cycles. However charging at 2C with todays batteries is no more damaging than 1C. The benefit if you wait half the amount of time for it to charge. I also notice more punch in my packs when charging at 2C for stock racing, mod no difference.