The gameplay is really good in theory. It's completely open-ended, the bosses are really challenging and force you to play around them. You have to actually try to get the right equipment and skills. The Trials are a great challenge. The initial story is pretty cool.
But at the end of the day it's the same dungeons with recolored bosses, though mechanically they can be quite different.
I thought it was really fun for the challenge and the open-endedness but it's meant to be a bonus thing for long-time fans, which is what I am. You get the DQ3 overworld music and battle theme, you get you-know-what, etc. It's bonus fanservice. There are really a ton of things about Act 3 that are clearly meant for veterans. If you are new to the series just stop at Act 2.
But like >>1386048
said they should have made a better job disconnecting it from the main story so that people aren't compiled to do it if they really don't want to. Personally I didn't really mind getting rid of the stakes and had fun "fixing" everything that went wrong in Act 2. Plus it's not like they didn't know this move is controversial, I liked seeing things from a new perspective too.
They tell you two or three times that you would be erasing the hardship and growth you experienced on your journey and if you really want to do that. If you said yes, it's your fault for "wanting to see more of the game" rather than get the story you want.