I've got an iPad which has it's uses, especially for reading, and for my nephews to play on. I used it a lot more though before I got myself a good 13" laptop (Macbook Retina).

IMO Surface is as good as tablets get. Almost chose that over the Macbook.

^this

I have an iPad 2. It served me well, but it's starting to show it's age.
Since I got a OnePlus One, I don't use my iPad as often anymore.

I got my daughter a Nexus 7 a couple of years ago but I never touched it. Never really cared for having a "giant cell phone that can't make calls".

Anyway, earlier this year, my phone broke. Verizon was giving away "free" tablets and I got an Ellipsis 7. Didn't even touch it for a few days. Then, I added a few apps, downloaded a few comic books (for free. If you want to know how, PM me!). Long story short, I LOVE it. It has low ratings from reviewers but it blew away my expectations.

I have multiple tablets.

I has a Ipad2 that I bought when it came out. I was traveling 5 days a week and had a work laptop that was very restrictive to corporate use. So, instead of carrying 2 laptops, a tablet was nice.

Since the Surface Pro came out I have had the first version and now the Suface Pro 2. THey are mostly for work, but they do all my laptop ever did and more.

I was going to upgrade to the Surface Pro 3, but decided to wait til the SP4 comes out. Just to see if they make the processor and gpu a little more powerful.

Also own a couple of Kindles for my kids to use. They are good for being toys, thats about it.

No, and I don't see myself getting one for a long time.
I don't like mobile games, I hate watching movies on a small screen, I read regular books, I already own a smartphone that I mostly only use online when I'm on the toilet (no joke) and I'm getting a laptop instead since I write a lot and want a better movie device that also plays PC games for working night shifts.

Initially got the iPad 3rd gen. Liked it enough to get an iPad Air. Kids use the iPad 3rd gen for Netflix, eBooks, educational apps and games.

I'd love to have one.
Then I ask myself....do I need it? I have a great laptop and a high end smartphone, and as much as I hate to admit it, I can live without a tablet.
So, no.

I had an iPad, and it was super convenient for personal internet/Neflix use as well as music, but now I'm doing a laptop-tablet 2-in-1 hybrid. I can't wait until it's perfected.

I've got a first gen Kindle Fire, and I've been using it for years. I love it for reading, Netflixing in bed, and playing the few mobile games I'm interested in. It's also hooked to my Amazon MP3 library, so that's nifty when I want to listen while I read. I know about a million newer better versions have come out, but it's still an awesome little device and I couldn't be much happier with the use I've gotten out of it.

As OP said, it's perfect for the treadmill! I'm sure if I had a smartphone I wouldn't need it though.