Pro:

• You don't have to move it when you change clothes - harder to lose (my son lost several before he gave up) or wash (which is what I did, although it did come back to life 10 days later).
• I don't need a chest strap HRM to get more accurate calorie count during cardio exercise.
• Automatic sleep tracking - no more pushing the button to start and stop sleep and no more hassle with the lousy wristband or some other way to attach it to the wrist.
• Always right there if you want to check it, just push the button or tap it - you don't have to pull it out of wherever you've tucked it. But... the display is hard to read in sunlight. (No surprise there.)

Con:

• Worn on the wrist. You can't discretely clip it to your bra. I hadn't worn a watch for years and don't wear jewelry because it drives me nuts, so wearing anything on my wrist bothers me. I'm gradually getting used to it.
• Doesn't detect steps very well when pushing a shopping cart (or stroller, I assume). I now turn off the heart rate monitoring and put it in my pocket when I go to the grocery store. Now that Fitbit supports having more than one tracker on each account, I sort of wish I still had my One. I could just use it when I go shopping, but I gave it away.
• Display is more easily scratched than on the One.
• Battery doesn't last as long. I haven't tested exactly how long it lasts because I have a convenient time to charge it every morning, but I know it drains faster than my One did. This is perhaps another way having a "backup" tracker would be nice. Just use it while your primary Fitbit is charging.

I think some people notice a difference in the estimated calorie burn and/or their step count when they switch, but mine seems to be about the same.

My hr has an incredibly high stair count. Must be some problems yet. My One works perfectly. I use both of them using different email accounts.

I went from a simple Garmin HRM watch (and chest strap) to the Charge HR. I love it. Constant monitoring of calorie intake all day long. No more uncomfortable chest strap. With the phone app, if I want I can get a gps readout of my walks/runs, but not necessary. I ignore the stair count. What matters most to me is calorie count, distance and HR. This does it all.

After a year using a One, I moved to a Surge (2 months ago) and after 14 months with a Flex, my husband moved to a Charge HR (5 weeks ago).

I received both of mine as a birthday gift from friends, so I didn't have to worry about the A$350 cost of the Surge, but if I had paid myself I would only have gone for a Charge HR too (A$199) as I don't believe the additional features (eg GPS) are worth the additional A$150. If you can afford the $350 easily though, then absolutely go for the Surge as it is an awesome device.

I'm a runner and I love the GPS function, so I do prefer the Surge, but that is about the only function that I use over the available functions on my husbands Charge HR. I like the bigger screen, but not the bulky/masculine look to it (especially when going out to dinner or something, in a nice dress) but I am yet to give in and take it off. I gave my One to my Mum when I upgraded, but I kinda wish I'd kept it for those times when I do want to dress up and not wear a sports watch

The automated sleep tracking is fantastic and I recently discovered that it also picks up random naps as well! You can't manually add in a second sleep to a day, so this is a great feature that the One doesn't offer.

The Surge can also control the music on your iPhone (which I don't use as I carry my phone with me while out for a walk or run) and you get call notifications and text messages to your device, which is handy if you don't want to stop to check your phone. This is not a function of the Charge HR (you can see call notifications, but not SMS and no music controls).

We both charge our devices up twice a week to ensure the battery never runs low, but I did come close to a dead battery a few weeks ago after using the GPS function 4-5 times in 2 days for 30-45 mins at a time. Both batteries have a great charge length though if not using any GPS.

I'm sure I am forgetting things, but that is the main points that I would pass on