Segment

Segment have built a best in class data layer and data management tool.
The tool is easy to set up and creating destinations and understanding sources is simple.

Add the code for Web, Mobile or a data warehouse. Then enable product and marketing teams to enable and disable marketing and product tools easily.

The new personas feature is also incredibly powerful.

Make collecting analytics .track and .page and .identify easy.

Segment does make data easy and because of the quality of the tool it does come at a cost. The benefit is it does scale with you because the price plan works on MTUs. But it really depends on the size of your business.
Beware not to over enable and connect too much if you are not using it. This will add un-necessary load which can impact mobile page speed.

Segment was easy to setup and integrate with our website, using the Javascript code they provided. Also the Segment interface is well designed and easy to navigate. The settings for your Destinations are all clear and laid out, so you can make changes effortlessly. The neatest feature is the visual tagger, which allows you to create and track triggers/events on your website without having to code anything at all. It's great if you want to track very basic behaviors like button clicks or form submissions.

The most difficult part of Segment was when we wanted to track more granular data, such as user identities/emails. I believe if you have a typical website, built off of WordPress, or Shopify, you might not have these issues. But if you have a custom built website like ours, you will need some technical know-how in order to capture the finer data. For example, visual tagger did not work for us when we tried to use the identify call analytics.js, because our website stored user ID info on our data layer, not the presentation layer, which is only what Visual tagger can see. In order to access fields in your data layer, you'll need to use something like Google Tag Manager (GTM) to trigger and fire those events in Segment (shout out to McGaw.io for helping us with this!). So, if you have a more complex website, triggering certain events or getting fine granular data in Segment is not intuitive.

Easy to use, amazing documentation, unparalleled functionality, and stellar support.

It's hard to find something to fault. We had to learn a lot about data structures, but Segment's documentation and resources made this straightforward. We're all the better for it.

Reduce the amount of code that handles analytics and tracking
No real alternative

Unreliable.
Bad customer support.