Offering the power (Guide Number 60) and features of Canon/Nikon Speedlites costing £300+ the Nissin DI866 Mark II represents excellent value for money. It is worth buying just for the ability to bounce flash from the ceiling and behind you for the dramatic improvement to indoor pictures; just switch the flash to Automatic or TTL and and press the shutter, everything is taken care of for you (its like a more powerful version of the pop up flash on your camera).
When you're ready, the DI866 Mark II offers full control and off-camera wireless master/slave options (Manual or ETTL 1 and 2) to take your flash photography to the next level (I haven't used the high speed sync mode yet so can't comment on that). I can also trigger it off camera optically using the pop up flash of my 550D (if you cover the camera flash with something like a used toilet roll to block its frontal light output the DI866 can be used as an off-camera key light similar to wireless mode - within limitation).
When mounted to my 550D the Nissin does add a little weight and bulk but this is to be expected. It possibly also marginally overexposes if pointed directly at subject but this can be controlled by reducing flash exposure a third of a stop.
Again, for the power and features the DI866 Mark II offers I don't think you will get a better flash (at time of writing) for £180.