Rpg Maker VX Ace is an excellent successor to Rpg Maker VX and XP. The new features and scripting are great!
Although I have only tested the trial of VX Ace, based on what I have tested it is definitely a much more powerful engine than both VX and XP. The new features such as the unlimited maps tilesets, weapon, skill, and armor attributes, the shadow tool, and much more will provide much more flexibility for your projects. Also, the RGSS3 scripting will open new doors for those who are looking to get the best out Ace. Finally, Ace comes with a large set of beautifully designed monsters. In my opinion the monster art is far better than any of the previous rpg makers.
In Conclusion, if you are looking for a solid program with many features and power, but yet simple to use, then Rpg Maker Ace is for you!