Yes, Memento is a really useful move, at least in competitive battling (what I play).
Memento is used when your Pokémon is at low health and/or you want to set up your teammate. Since Speed is an important factor, Latios and Scarfed Uxie (trick) are probably the best Pokémons to do so.
I have had a good history with this move, especially with Scarfed Uxie:
- I come in after the opposing Pokémon knocks out my Pokémon.
- I use a quick Memento to weaken them.
- I send out a Pokémon that resists the opposition's attacks.
- I set up.
- I have fun sweeping a few Pokémons.
Important thing to note:
Timing is key. Memento is very similar to Healing Wish - use it wrongly, and that Pokémon is wasted. It might go wrong at first, but after a few tries you will be experienced, and you can start sweeping.
Good luck then!