Field Magic cards generally weren't thought of as very good, as they have to sit on the Field, and most aren't even worth destroying, because (at least in the beginning) the general attack boost for a type or attribute wasn't enough to tip things in your favor.  But there are a few that are playable, most notably things like A Legendary Ocean, Skyscraper 2-Hero City, and Ancient City-Rainbow Ruins.  The group of useful and playable Field Magic cards goes up by one with U.A. Stadium.
 
The first effect, at least in principle, does slow U.A. Monsters down.  You have to Normal Summon a U.A. (and you know that by now they gain their speed through Special Summon) but in using a Normal Summon you can add any U.A. you choose from the Deck to your Hand.  That, alone, might not be enough to warrant being played in the Deck.  But the second effect does.
 
As for that second effect, if a U.A. is Special Summoned to your side of the Field, ALL Monsters (not just your U.A. ones) gain 500 attack.  The fact it encompasses any Monster you're playing makes that a whole lot more worthwhile, as does the fact most of the U.A. have moderate to low attack, so the boost is greatly appreciated.  

Now, the concern I have, and I did some digging but I couldn't find a ruling on this...the card reads (or at least I take to read) that EACH turn you Special Summon a U.A. your Monsters will gain 500 attack.  It says once per turn, nothing about only once, but I did read that even if U.A. Stadium leaves the Field, all your Monsters that gained an attack boost from this card, the boost doesn't go away when Stadium leaves the Field.  Regardless, this can make a quick Deck even quicker, and I'm very in favor of this.
 
Rating:

4/5  You want this about as soon as you can get it in the game, and it does become less powerful mid to late game.  But very good for a Field Magic card.
 
Art:  4.5/5