xerath is a pretty good champ and is more than just "playable." he is tricky though becaus all of his abilities are skillshots and he is rather imobile.
what you want to do as him is take advantage of his long range, that being said you should also be looking to hang back in fights and deal solid damage from afar. in lane you should be looking to hit as many things as possible with his abilities, now i dont mean spam them, i mean if you are going to use your q to hit creeps, do it when the enemy laner is in the creeps to get the most from each cast. if you are playing in a draft type game mode, i would not reccomend blind picking him, as he is very counterpickable.
when you are playing xerath you should also be looking at what summoner spells you bring. as you may or may not know, most mid lane picks are going to be running flash and ignite/barrier, xerath is one of the champions that you are going to be wanting to run barrier. this is because you are going to be looking to pick people off at a range and you are rarely going to be in range to use ignite, also because you are not necessarly looking to kill everyone as xerath, granted kills are always nice, but as xerath you are going to be looking to stay alive and do as much damage as you can in fights.
i hope i helped, if you have any questions feel free to ask either here or in game!