situational. He can be great, but it depends on your team comp. He has great aoe damage for a teamfight and poke, but no mobility or CC(it's just 1 short single stun) --- Idealism is seeing potential. Pragmatism is seeing consequences.
He's good for arranged teams, terrible for soloq. Unless you're rediculously overfed, Xerath excels in pre-fight poking and relies on peels to live through the engage. In solo Q, everyone just charges in before you can really poke so most of your use goes out the window.
as a xerath player i would like to say....... LB breaks him badly Strong in good comps, countered by silence, high skill cap(took me at least 100 games to master the range of his W with smartcast) need blue