The 6.5 GS review score for PS All Stars Battle Royale is absolutely asinine
First, the biggest WOW factor" for me is the cross-play/cross-buy between the Vita and PS3 version. This is an idea that is fantastic. Not only is the value of the purchase significant, but the whole idea is something I've been waiting for awhile now.
The character roster is doable. It is not going to satisfy everyone. You will always have those "where is this character?" and "why didn't they include this character?" people, but the amount of playable characters is sufficient for a first go-around. The characters feel like you took them directly out of their original games and put them into the battle arena. What I mean is that they characters actually feel like they do in their own games, which is something I can't say for Super Smash Bros.
The story mode is lazy with really no substance. It's the biggest detractor. They had a huge opportunity to blow any Super Smash comparisons out of the water with a superb single player story, but entered the same pitfall and ended up actually considerably worse than its prime competitor.
The unlockables are great, but still not as solid as Super Smash. I had spent many hours with SSB Brawl that had more meaningful and varied unlockables. The games biggest gameplay difference is the Special Gauge. Once this meter fills up, you perform your special which is the only way to get kills. This style will grow on you, but it makes character selection more heavy on their specials than the normal combat moves.
The game seems to focus on online play, which is definitely fine by me. It's fun and connection is pretty seamless.
Overall, I'd definitely recommend it.