I've had Makita's 18v line for coming up on ten years. Dozens of tools, about a dozen 5ah batteries (the old 3's all died in the last couple years). They do what I need them to do. I fabricate and install scenery and displays. Wood, metal, plastics, etc.
I recently picked up one of those 5 tool M12 kits on a whim, because I've been wanting to get a smaller 12v drill for doing installs, and I really wanted to play with the 3/8" ratchet.
I was definitely underwhelmed by the brushed drill and impact.
Then I went and spent even more money, picked up the M12 brushless drill and impact kit. I am quite impressed in the couple weeks I've been using it, and I can already see the impact driver becoming my main driver. It's just plain awesome for me, and I would highly recommend it. It has plenty of speed and power for most uses, and it's just so much smaller and lighter than any 18v option. I could carry it around on my belt in the shop all day and never feel like I wanted to put it down.
So to answer your question, I did just fine with only 18v tools for a long time, but now that I have both, I plan to keep both. Each voltage has its place in my toolbox. I don't plan to get a 12v circular saw, but a tool like the impact driver I'll use every day.