I worked at a company that used Softlayer before we were acquired and before Softlayer was acquired by IBM. We moved off to the acquirer datacenter, because that's what you do.

Softlayer was a very nice place to get bare metal hosting. You get a whole machine to do whatever on, they provide a solid network, and replace components when they fail (and you open a ticket). We didn't use any ancillary services for the most part, we had been using their included DNS, but switched off because update delays were getting too large and we didn't like the support response on that; we used their loadbalancers for one service for a few months, but it had worse uptime than the hosts behind it. I have heard their other ancillerary services were (are?) bad too. From what I saw, support in general was getting worse, but that could have been because of IBM takeover or because we were going from a top ten customer to an average customer.

I would consider them an option in cases where you want to run all the services you consume, but you don't want to run your own datacenters. They had a decent worldwide footprint, and post-IBM they got a lot more datacenters outside the US; that could be a big deal, because sometimes you just need a couple machines in another continent, and it's easier if you already are on board. Also, at least when we were there, private network traffic was free, worldwide, which is nice to manage replication for disasters.

If you're more cloudy though, and make use of integrated services, and small instance types with per minute billing, I wouldn't consider it. Also, they had that global routing issue earlier this year, which would have been hard (impossible?) to mitigate as a single vendor customer.