What does the software or service do well? Salesforce is a platform - not just a piece of off the shelf software. What does this mean? You can extend it to do what you need. For us, we were able to consolidate pre-sales, sales, marketing activity info, post-sales, all of our customer KPIs from our SaaS Platform (we're a software company), stripe charges, stripe refunds, stripe coupons, Intercom conversations, Freshdesk tickets, customer service activities, how much we charge, etc. into a single point of truth. This is pretty priceless.
What does the software or service not do well? Salesforce is a complex beast. Out of the box, it will mostly focus on doing Sales CRM. You need to do a lot of work customizing it and connecting it in order to make it a single point of truth for everything in your business. Some customizing is drag and drop. Other customizing requires a knowledge of coding - and using their proprietary language. And it sometimes shows through that they are a legacy platform when you can't do stuff that doesn't make sense. Also, it's not an automatic transition to move from their classic to their lightning interface.
What do you wish you had known about the software or service before using? They are still transitioning from their classic interface to their "lightning experience". Lightning is anything BUT lightning with regards to speed, but it does look a lot nicer. However, there are still numerous things you can't do in lightning that you CAN do in classic and vice versa. Without knowing exactly what I wanted to do in Salesforce it was tough figuring out which interface to invest into (we ended up with Classic and are slowly moving everything over to lightning).
Have you used a competitor to the software or service? If so, how does it compare? We used several CRM only solutions prior to Salesforce. Anyone who thinks that Salesforce is just a CRM is sadly mistaken.