In reply to mypyrex:
Best time for the Lairig Ghru is probably early spring, after the snow is gone but before the midge explosion!
The most difficult section is probably the rocky / bouldery section before the pools of dee (from the Aviemore side), though it is just a rough path, nothing hard. On the whole it's a good track all the way.
Best direction will depend on your transport options! If you are public transport only, it's probably easier to start in Braemar, and walk through to Rochiemuchtus. That way you don't end up with the 6 mile trudge from Linn of Dee to Braemar at the end of your day! Plus you can finish up at the Glenmore Hostel which provides food if you book in advance so no worries about carrying food for evening meal or accomodation at the far side. That that puts you a quick bus trip to Aviemore and the A9 / main train line.
Or check to see if the heather hopper is back running (linn of dee car park to Pitlochry) as this would drop you back on the main train / bus routes. Not sure if it's run in recent years though.
If you feel enthusiastic enough the 2 day version suggested by Doug is a great way to start / finish back at your car. You can camp at Derry Lodge area (very nice, but can be midgie in the trees), use bob scott's bothy (don't assume you'll get a decent night's kip though!) or keep trudging up to the Hutchie for a long day, but more likely a quiet bothy night in a fantastic location (stove, but no local fuel source). Also opens up the option of returning via Macdui & Cairngorm (if you want some extra ascent though quite possible winter kit will be required) or via the shelterstone and the head of loch a'an (just stunning scenery).
Cheers,
Stu