My experience thus far with Mintsy has been twofold: a purchased SHA256 mining contract (1TH/s for 90 days) and pointing my own hardware here.
First, here is the good news. The mining contract which I purchased was given a boost to 2.5TH/s and extended. This was an unexpected surprise and I'm very thankful for it. I've actually got a relatively decent chance to come out positive now.
The UI is easy to navigate and you can switch coins at will. If you've got a cryptsy account, you can hook it up directly using your trade key. Any alt coins you mine get paid out to your cryptsy wallets automatically. Bitcoin mining payouts are a manual process. Stats are plentiful, too. You can look at your average hash rate, see found blocks, pool hash rate, expected time to block, etc.
On the hardware side, I've had issues aplenty. Since the pool is also listed in the alt coin service announcements, that's where I first reported my problems. You can see my posts at this link:
https://bitcointalksearch.org/topic/m.11256333.
My main concern is that when my gear is pointed to this pool via MRR, it fails. Even though I've got the latest version of cgminer with the nicehash #xnonce patch as required, it simply does not connect from MRR. Workers are constantly restarting or refusing to hash and fall back to secondary pools. When I connect my gear directly to the pool, things work as expected.
I have not attempted to connect to and mine external pools with either my contracted hash or my own gear, so I don't have any comment on others' experiences doing so.
Huh... I wonder if I could setup an infinite loop. Here's the scenario:
I setup an external pool here for MRR. On MRR, I setup my primary pool to point back here. So, I'd be telling Mintsy to mine on MRR and MRR would say to mine on Mintsy. I might try that for fun after my gear's current rental ends (assuming it's not immediately rented again).
That makes me think of another scenario. If a purchased contract can be pointed to an external pool, could that pool be MRR? I'd be able to rent my contract out on MRR. Another thing I might try.