I like the idea of the bikes, but I haven't played much with bikes, so its hard for me to say how well it would work out. If you get them too far ahead of the Harlies, you may make it too easy for the enemy to step sideways and shoot around the ends of your line, but as long as you're careful about that you should be able to deliver the Harlequin payload pretty quickly.
I'm not a big fan of the ablative Guardian shield. I don't like taking something for the sole purpose of dying, and I don't really see those guys doing much else without a transport for delivery, which would make them useless as a shield. Overwatch from 15 Guardians sounds awesome, but its probably only 9-10 by the time anything in 40k charges them, given even the most desultory shooting first. That leaves you what? 3 hits? At S4 with normal armour saves? I'd charge that. The bigger deterrent to charging them would be the counter charge from the Harlies, of course, and you might be able to cause some significant damage with that if your opponent takes the bait.
Personally I like Harlequins behind Wraithguard. The WG shield is hard as heck to kill. If you charge it, they eat things in Overwatch just because nearly every hit is a kill with WG... and then they're tough in HtH... and then you cop a Harlequin counter-charge to boot. If you don't charge it then you sit there taking withering fire from the WG instead; never a lot of fun. If you dive-bomb it with flyers, best stay at least 18" away from those WG - doesn't take too much luck for them to down or disable a flyer. If you stand back at long range and hammer them with heavy weaponry, they die. But they die slowly, while absorbing an awful lot of heavy weaponry that could be going somewhere else, and all I need is one decent piece of terrain to leap the Harlies forward to threaten the guns. The WG as a troop choice can take and hold forward objectives, while the Harlies zip forward from there to disrupt enemy-held ones.
Its also a stupid-expensive core to my army, and its slow. Many armies on many maps can just stay away from it altogether, though that is generally hard work and cedes a big chunk of map to me. Its by no means without counters, but it is pretty awesome, and demands some sort of reaction from my opponent every time.