Ironically, if the employer mandate wasn't delayed a year (still don't know what was up with that), it would seem to me that Congress could have been fined for dropping coverage for their employees upon the ACA go-live.
Congress is the only employer that is actually required by the ACA to drop their existing coverage of their workers and require them to purchase their own insurance (and contrary to popular belief, you don't have to purchase your insurance on the exchanges; that was just supposed to make it easier - although so far that isn't the case - and would be the only way you get the subsidies if you were eligible for them)
All other employers (above 50 employees) are *required* to provide health insurance to their employees (although enforcement has been delayed a year). So yes, Congress got "exempted", but not in the way the ACA-haters are making it out to be. The "exemption" was actually put in by Charles Grassley, a republican, because he thought that this would kill the bill. However, congress actually said "sure, whatever, we don't have a problem going through the exchanges just like all the people who don't have coverage now". The "exemption" actually requires these employees to get their insurance through the exchanges (or on their own if they want), rather than to just stay on their employer's group plan like most other full time workers in the country.
The only remaining debate is whether to take the money that Congress was previously kicking in as a contribution to their employees' group health care and add it onto their employees' paychecks instead, which seems fair to me.