Inb4esamupdates, maybe?
First Transformation (Too Freakin Small Castle Roof):
I agree with Kitt.... this is the worst out of the three for Pika. It's a little too cramped for Pika's tastes, and combined with the small side blastlines, stronger characters who love close-mid range combat will make it hard for you to stay out of their range. The platforms are also a problem, because characters like Marth and MK can harass you from below if you happen to land on one..... it'll be hard to get down without eating considerable damage.
Traditional campers hate this part, though, while Pika doesn't have to worry about his projectile being compromised. The slope in the middle can aid in CGing, and several recoveries have a tendency to get gimped by the almost-invisible lip on either side of the stage. This can hurt Pika as well (think Yoshi's Island), but with stage familiarity and careful aiming, it shouldn't be a problem.
Edgeguarding works well here because of how close the sides are. Nair, dair, and thunder off-stage kill considerably earlier than normal.
While Transforming ("Grey Stage"):
If you've grabbed someone and the stage is in the process of transforming (either to or from the "grey stage"), you'll automatically release them. As far as I've seen, it's always been an air release, but that could just be a coincidence.
Also, characters near the edge of the stage at the moment of transformation tend to die really early even if they're out of hitstun and in the process of recovering. Use this to your advantage, and try not to get hit with a potential KO move right before the stage starts to change.
This part is good for CGing and bthrow shenanigans. It doesn't last long, so you have to be quick about it, but there are walk-offs on either side. Otherwise, it's just flat.
Second Transformation (Statues):
There are walk-offs (two on the bottom, one on each of the two top platforms). Abuse them.
The bottom two platforms are ideally spaced for uairs, so if someone's above you, try to keep them up there for free damage from below. The top platforms are high and good for thunder, but I personally dislike actually going up there since it's hard to see what either character is doing. I do like the fact that the bottom platforms aren't stacked one right over the other..... you can walk off > fair/uair people from the top one to the bottom one, which tends to catch them off-guard. Similar to the first stage, though, be careful of characters who have good platform game; the good thing about this transformation, however, is that you can always move to the bottom and have enough space to get away. These platforms also aren't as low and detrimental to our aerial game.
I think everything's been said about the statues themselves. Jab them to refresh your other moves; they extend hitboxes; tjolts will be eaten by them. Don't waste your time breaking them.... the transformation doesn't last long enough for it to really make a difference, and they don't get in the way besides cutting down on your long-distance camp game.
Third Transformation (Tilting FD):
Shorter FD that tilts. I've had the tilting gimp me as other characters (read: Zelda), but it's never happened with Pika and really shouldn't with our versatile recovery. The only thing that might get you is if it tilts upward and you end up QA'ing against the bottom instead of to the ledge..... you'd have to be pretty careless for this to happen, though.
Dip in the middle can help or hurt you depending on how you use it, but I honestly don't notice it enough for it to make a difference to me.
This part gives you all the benefits of FD with smaller sides, so you can essentially still camp, but killing is made much easier. I really like this part for Pika. As the stage goes back to the first transformation after the "grey stage" following this one, try to knock people near the sides; as Kitt explained earlier, you might get a stage gimp.
Character Picks & Overall Rating:
Characters I'd take here: Snake (possibly -- BEWARE TILT **** ON FIRST TRANSFORMATION & hidden explosives on dark background), DDD, Ness + Lucas (watch out for extended PKT2 on statues), Falco, Wolf, Sonic, Ganon, Pit, possibly Fox
Characters I wouldn't take here: MK, Marth, Kirby, Jigglypuff, Ike, Zelda, Link, Luigi, DK, G&W (if you're blind.... default color is so hard to see here....)
Overall Rating: 7/10. It's pretty good for Pika, but it's not amazing. There are better stages to go to for nearly every character you can bring here.