Customer.io

This tool is well-suited for companies looking to run email marketing campaigns or send out newsletters regularly to their users/customers. The workflow setup is especially useful for companies who want to send specific emails based on user behavior - i.e. they've not logged in recently, have bought a specific product, haven't opened any emails, etc. This could also be useful for freelancers like me since you can create different workspaces within Customer.io. I have configured it this way to handle email marketing for a few of my clients, and the software keeps everything organized and separate. This might not be the best tool for small businesses, considering the price. It may be better to start out with a cheaper email marketing tool, and then if/when necessary, scale-up with a tool like Customer.io.

If you are sending email campaigns or newsletters, I have yet to find a better solution. You can easily tie all of your systems into Customer.io, which makes segmenting your customers based on location, preferences, activity, or any data point you measure.

We are just starting to use SMS and it isn't using a native system but tying into Twilio, so I think you would be better off with a dedicated SMS platform if you are focusing heavily on that channel.

Customer.io is great for teams that want to deliver highly personalized messages to the right people at the right time.

If you want to run a fast test campaign for let's say a new product, or collect data really fast and turn those in easy segments to communicate in a better way. Then go for it, it's so easy and fast to set up a customer communication stream, that it's also great for testing smaller campaign. Most of the time the hold back would be time + setup to do this. Not with Customer.io.

If you have a very small database of people, then probably the whole integration isn't justified. If you have a small business and don't plan to scale much, don't do it. It will be expensive for what you can get elsewhere (Mailchimp).
If you want a scalable and powerful platform, that will take some time to pay off, and you can wait, then do it. Once you get integrated and start sending events and attributes, the things you can do are almost limitless.

Customer.io is great if you're a B2C company, or you have to send out lots of automated / triggered emails based on customer behavior. If you want more advanced automation workflows (e.g. B2B marketing) then you might want to look for a different product. Either way Customer.io is a great email tool.

A few specific scenarios where Customer.io is well suited would be attempting to scale your email campaigns, personalize and trigger emails based off of events from your system. Companies who use Segment and other third party tools would be well suited as Customer.io integrates super easy with other tools.

Where Customer.io is not as well suited would be an environment where each lead/customer requires a personalized email/attention. Customer.io is designed to help you scale, but if you are just writing one off emails you might like another tool.

Customer.io is perfect for drip email campaigns, meaning any scenario where you need to send a series of emails to a certain client. Ideal for engaging new prospects, maybe ones on a free trial for your own product, or for upselling clients based on their activity on your own site or their engagement with your own products. Also ideal for onboarding and tutorials/training emails.

It's perfect if you want your email series to change dynamically based on customer engagement: for example, let's say you get a new client and want them to complete 3 key steps to register with you. If they take Step 1, but not Step 2 and 3, you don't want to email them asking to do Step 1. So Customer.io would automatically skip your "Step 1 Email" and move on to Step 2 and 3.

It's also excellent for A/B testing, so you can optimize whatever you're using it for.