Several points above are not true in SB anymore. The DRK got changed massively in the update, and turned from being (intentionally) the busiest and most complex tank to being the least complex and not really busy anymore. It also suffers from several abilities being outclassed and made obsolete by others for most situations.
Darkside does not kill your MP anymore. Also, since the job lost most of its OGCDs, it is not high APM at all. Its pretty slow and steady, like WAR before stormblood, but with less buttons to use.
Being a good DRK will require you to know which half dozen abilities to ignore most of the time, and being happy with using one combo over and over on single targets. I am playing all 3 tanks and the new Samurai, and can say without a doubt, DRK is the easiest to master because it just lacks depth. There are no decisions. If being able to play "perfect" relatively easily appeals to you, by all means give it a go. But right now I consider DRK to be badly designed, or probably not really finished (you can already see in the order of abilities gained that the design team forgot a last QA pass, because you get your gauge and can basically not do anything with it for a long time.).
