My veiw on Xerxes is that he is a very powerful leader.
In the play The persians by Aeschylus, this is often demonstrated. Though in the play they demonise him with his power (as the text was written by an Athenian Playwrite for Greek entertainment) I think that it was important in running a stable empire. The way he dealt with the results in the empire also demonstrates this, the punishment he use, though very harsh, established this power and stopped the revolts getting that significant in it's impact on the rest of the empire.
Though not necesseraliy a "good" man (he could be described as sadistic, arrogant, gullible and having hubris, these veiws are definitely supported in Herdotus' account but it must be remembered the bias he would have, and the fact Herodotus also wrote to entertain) he was definitely a powerful leader.