I don't really like the Mini, other than you can upgrade to the screen of your choice/size/resolution...it being separate from the screen is more of a liability to me. I think I'd rather have a AIO or really a redesigned smaller iMac.
To me, the fact that it lacks a built-in monitor is it's strongest selling point.
The lack of an Apple monitor being built-in, is the only thing I really like about the mini (well, it's a fine machine, just a poor price for what it is).
Once you've been through as many iMac screen issues as I have, you learn to avoid the iMac like the plague. The iMac would be an ideal machine for me if it did not include a built-in monitor (given my history with them).
I've had enough iMac screens die (which negatively affects the machine's value and negates it's all-in-one purpose), to know that a built-in monitor is really a weakness.
So, when I was in the market for a new machine, I looked only at two models. The Mac Pro and the Mac Mini.
The mini was over-priced for being a lower-spec entry level machine. The Pro was over-priced for my needs (being much more computer than I needed).
But, given only the two options without a built-in monitor, I went with the Pro.
If the mini had a dedicated GPU and a 7200 R.P.M. hard drive, I would have purchased the mini.
Really, even with it's lower specs, the mini is a better machine from my perspective than the iMac simply because the mini lets me supply my own "good" monitor.
And, if the monitor I have dies, I just get another one. I am not stuck paying Apple's premium for an out of warranty monitor repair (assuming they have some available), and the lack of a monitor doesn't affect either the Mini's or the Pro's value (since they are expected to be without a monitor).
I was fortunate that all my dead and defective monitors were covered by Apple's warranty. But, if I had run past the warranty period, it would have been a very costly repair (about $900 for each replacement screen).
The benefit of an external monitor, is that if it dies, you just replace it for a low price between $50 and $200. Much better than $900.
And, as an added bonus, you don't lose the use of your machine. For example, if I had an external monitor and a mini, I could just replace the screen same day (with a new one). Or, if it were under warranty and needed to wait for repair, I could use either a loaner or a cheap $5 thrift-store monitor to keep using my machine while I waited for the replacement monitor to arrive.
In the case of the iMac, I was without a computer at all for the couple weeks that I had to wait for Apple to come up with a replacement panel. Since they had the whole computer, all I could do was wait until they got it back to me.
But, if it had been a two piece unit, my downtime would have been eliminated.
It's kind of like the television with a built-in DVD player. When you take the DVD player to the repair shop, you lose the television as well.