Xur's Ophidian Aspect is definitely one you should grab--it sports a stat roll of 66, which is above average for the vendor, with high Intellect and Resilience.
In the Legendary weapons department, there's not much to be excited about. Xur's offerings are pretty middling. If you'd like a decent sidearm, Xur offers the Seventh Seraph SI-2 with Full-Auto, Dragonfly, and a Range masterwork. His version of the Seventh Seraph CQC-12 shotgun isn't bad either, with high Range to go with Slideshot, Vorpal Weapon, and Handling for its masterwork. But neither is especially exciting, nor are the rest of his guns unless you're really hurting for something.
Much better, however, are the options for Legendary armor. Xur offers the Future War Cult's Simulator set from the Season of the Worthy. Hunters and Titans can really make out here. For Hunters, there's a chest plate with a total stat roll of 64 you should nab, plus a few other items that drop at 61 or 62--enough to make yourself almost a complete set of armor at decent stats. For Titans, you'll likely want to pick up the helmet and chest armor at 63 to shore up your loadout as well. Warlocks have slimmer pickings, unfortunately; there are a couple of pieces around 60, but most are below and you'll likely already be using better stuff.
Xur is present every weekend in Destiny 2, starting with the daily reset at 9 AM PT / 12 PM ET each Friday. His exact location is always a mystery when he first arrives, as he is not listed on the map, and for novice players, he can be easy to miss. However, there are a set number of locations where he takes up residence, including the Tower Hangar area, on Nessus in Watcher's Grave, and in the Winding Cove area of the EDZ.
Alongside changes in his location, Xur's inventory also rotates weekly. That means it's worth visiting him each time to check out his new weapons and rolls on Exotic and Legendary armor. You can visit him any time between his arrival Friday and the weekly reset at 9 AM PT / 12 PM ET the following Tuesday when Xur departs the solar system.