It's not like the After Hours party that takes place just in the evening and there is a limited amount of tickets sold so that the crowds are really low compared to regular hours. As I understand it (since it's my first time attending as well), those with MVMCP tickets can get in at 4:00 and it can get crowded by official start time. Rare character meets garner long lines as well and rides tend to remain the same wait time as normal hours. But a lot of people still have a great time despite the crowds.
You really have to pick and choose what you want to do between shows, rides, and characters not to mention the treats stations (again long lines). If I were coming back next year or the year after (kind of need a Disney break after this XD), I probably wouldn't have gone all out on two parties. But because I want to make sure I see the shows and the rare characters (still questioning how long I want to wait in line for Jack Skellington), I decided to get two parties and even pay extra for Fireworks Dessert Party and Tony's Town Square (that was a REALLY tough decision to make considering it's adding even more to that cost).
Whether it's worth it is pretty much up to you if you want to see the different shows, rare characters, and various treats or if you would rather use that money for an extra day in the parks since the crowds are going to be at that level anyway.