We recently compared a handful of remote I/O systems, including IO-Link and Murr's Cube67, to try and standardize on a method for doing our field wiring for machinery. In the end IO-Link was the winner for us. Some people were surprised to hear that we considered IO-Link for remote I/O (mainly salespeople hocking other systems). It seems to be more known for "smart sensors" and such, but we found it to be a very capable contender for I/O expandability.
To me, one of the biggest advantages of IO-Link is that it uses standard non-shielded M12 sensor cables for the drops to the I/O boxes. We started stocking all the different common lengths of double-ended cables, so when you're working on field wiring you just grab the lengths you need. One disadvantage is that you can't daisy chain devices, i.e. all slave nodes have to be in a star topology from the master. (There is one exception to this... some of Balluff's nodes allow you to daisy chain one additional device.) With proper planning of the IO-Link master hubs and your top level fieldbus, I don't see this as too much of a restriction.
My biggest beef with the Cube67 system is that it's completely proprietary. It's very flexible as far as topology and layout, but you're completely locked in to Murr for replacement parts and expansion. The connection between nodes uses 6-pin M12 cables which, although not unique to Murr, are certainly less common than 4-pin and more expensive. One of our considerations was, if we need to expand the remote I/O network in 1/5/10/20 years, what platform is more likely to still be available? I expect Murr will still be in business but will they still be making a compatible product line? With IO-Link you aren't locked in to a specific brand. So far we have mixed and matched Balluff/IFM/Banner/Beckhoff without issue. (Oddly, the one IO-Link device we did have trouble with was from Murr.) I've heard people say IO-Link is a fad but that isn't my impression.
Let me be clear, IO-Link isn't a replacement for a top level fieldbus. We are using EtherCAT which is super fast but we didn't want to run Ethernet and power cables everywhere. In cases where IO-Link doesn't cut it (say, for a high speed sensor) we can always fall back on EtherCAT and just deal with the extra cabling. But for us this is the exception, not the rule.