I've had a Pure Freedom system for 5 years. Would I buy the same again. Yep, in a heartbeat. It's earned me tens of thousands of £ in that time and has been totally reliable. I've replaced things such as resin, reels and hoses, but the system is the same, including the RO membrane.
What have people got against Whydale tanks?
To consider DIY I do think you have to have been in the business a while, so you understand just how everything works and what you need (10" or 20" filters? What size DI vessel, RO). If you know your needs and are handy (which I'm not particularly) it could be done.