How will Deoxys be?

Psycho boost isn't going to be anything like yawn? It's not a fast move for starters. However It does seem like a kinda weak move, but that could change when Deoxys is released. Don't forget that Deoxys had different forms in the main series games too, so we will have to see what happens with that.

Also it's a mythical pokemon... they haven't even released a mythical pokemon for gen 1 or 2 yet, yet alone gen 3.

Pyscho boost according to GamePress: https://pokemongo.gamepress.gg/pokemon-move/psycho-boost

All of its moves are similar stats. Psycho Boost on it is slightly better than Swift with STAB and comparable to Zap Cannon, though it looks like Zap Cannon will be the better move.

I could see it as a viable glass-counter to Lapras with Counter/Zap Cannon. Counter without STAB is nearly identical to ZH with STAB (ZH has a slight edge but this would be hardly noticable at a mere 0.1EPS and 0.31DPS).

Based on the information we have available, currently it looks like Deoxys will be the glassiest of glass cannons with an attack stat that soars above everything else, coupled with very low bulk.

That said, it's currently listed move pool looks highly unimpressive. Specifically it's signature move, psycho boost, with it's long animation time (4s) and late damage window. That, paired with Zen Headbutt, results in two slow moves, which aren't exactly ideal for a glass cannon. If had to guess, I'd say that it would likely get outperformed by both Alakazam and Espeon as a psychic attacker.

Also, correct me if I'm wrong, but I think Deoxys is considered a "mythical" Pokemon (in the same vein as Mew and Celebi), so it'll be interesting to see how Niantic eventually introduces them to the game.

Currently only the normal form is in the game data. If they introduce all four forms, their base stats would be:

Normal: 345a/115d/100s
Attack: 414a/46d/100s
Defense: 144a/330d/100s
Speed: 230a/218d/100s

Attack form is even more of a glass cannon. A potentially soloable T5 raid, assuming it doesn't just obliterate all your attackers.

On the other hand, defense form as a raid boss would have the same tankiness as Lugia (because all T5 have the same HP), but lower attack.