Firstly let me say, that i have been running a hosting company for going on 8 years. In that time i have used Modernbill 3-5, and whmap. 
  
whmap just was to featureless to seriously consider running a business on, and well Modernbill was the most buggy, hard to use, crash prone, expensive, no tech support offered package we have ever seen. 
  
What i can say about WHMCS.... 
  
1. it WORKS, flawlessly it seems. Besides a few minor issues with email piping and other easy to solve stuff, this product does what it says it does, and does it extremely well. 
  
2. It has almost every feature i can think to ask for with the exception of one, which im so surprised it doesnt have, that i will devote a paragraph below to it. 
  
3. It is VERY user friendly, both for my clients and for my staff, and for myself. 
  
4. It was EASY to install and setup. I never even had to read the installation manual or user docs! 
  
5. With the exception of a few minor bugs, it was SIMPLE to import my clients from Modernbill. 
  
Now, since i dont believe in saying all good about a product, and no bad.... 
  
  
1. So far i have yet to find a way to change the word "Linkpoint" on the customers Invoice, or the word "Linkpoint" on the email templates to "Credit Card", ill have to say this is a bad thing, though im sure theres an easy fix. But i mean, cmon, if a customer pays by credit card, regardless of the gateway, then the invoice drop-down and email templates should say "credit card". Customers have no clue what "linkpoint" is and will not select it since they dont realize its a credit card terminal. 
  
2. So far, if a customers "product" is canceled (his hosting account), he still gets a new bill each month. Im sure there may be a config option for this, but canceled packages should not continue to be invoiced. 
  
I know what your thinking, this really isnt even worth complaining over, and your right. The whmcs is AMAZING in my experience, and i would not use anything else now after trying it. However, if these two small points got resolved, i would give this software a 10/10 stars if anyone ever asked me about it. So for now, its a 9/10   
  
  
Sincerely