DHH's arguments about "opinionated" software seem to have been lost on the Merb crowd.
I don't think Merb will replace Rails. I think, ultimately, the two appeal to different crowds. Too some extent, I hope people who are happy with Rails stay with Rails. Merb feels like it's more aligned with the Ruby community than the Rails community. If that makes any sense.
I switched to Merb because of Slices. But Datamapper turned out to be an amazing upgrade from ActiveRecord. And I'm starting to love all the other little framework features (defer blocks in routes for example) that just don't exist in Rails.
I found rails to be really great for developing small applications but I find that any non trivial rails application start to have quite a few hack to get rails to play the way I want
What I'd like to do is take those model-like classes and persist them to the database. It should just be a simple matter of mapping a few instance variables / attributes to database columns right? Well, it turns out this is fairly difficult with ActiveRecord. Firstly, ActiveRecord wants all your models to extend ActiveRecord::Base. But, I already have an existing class and the chances are I don't want to, or can't, change the existing class to extend AR::Base.
So maybe I'll to create an ActiveRecord model that will wrap and delegate to the existing class. Ah, but AR::Base has a huge footprint and uses so much magic that it's downright painful to do delegation. You can't safely use method_missing because AR::Base does, and you're almost guaranteed to have a conflict where AR::Base and your existing class define the same methods anyway.
Here's how DataMapper is different: You're not required to extend any sort of base class. Instead, you mixin a module with a tiny footprint. So, in your web frontend, you can open the existing model-like class, mixin DataMapper::Resource, and map your properties to columns. Because the footprint for DataMapper::Resource is so light, and it doesn't use any magic, it's unlikely we'll break the existing class. Even if we do break it with this approach, we're far more likely to succeed with delegation that we are under ActiveRecord.
So, to bring it back to Merb and Rails, here's the key difference: ActiveRecord might be just fine for most projects. But, maybe DataMapper is sometimes better. Or, maybe another ORM is. In any case, Merb will let me work with whatever library I think is best for my project. Rails give you no choice. You either take the whole stack as is or you don't.
And, that doesn't just apply to the ORM layer. The same can be said for template engines and javascript libraries.
That said, I don't see anything in Merb that would tempt me away from Rails for new projects.
We can all live as one. It doesn't have to be "Merb versus Rails". It doesn't have to be a war.