I received the Evo 3d on Wednesday and have used it for about 7hrs total since then. I had (and still use) the Evo 4g prior to that. I spent a good amount of time just comparing the 2 focusing on the things I use my phone for and these are my thoughts:
BODY:
I love the slab/slate form factor of the Evo 4g but I love how the Evo 3d feels in my hand. I like the textured back and it feels a bit lighter. It is not as wide as the Evo 4g which is nice, but I have bog hands so doesn't really matter.
Contrary to some reviews, I like how responsive the power button is. Overall the Evo 3d body is nice looking.
LockScreen:
The ability to go directly into apps from the lockscreen is a major plus as far as usability goes for me. I have Mail, Maps, Internet and Messages as my for lockscreen apps. These are the apps I use the most and the ability to open from the lockscreen is awesome. This is my favourite feature of the phone.
Keyboard (Swype)
The higher resolution has made the swype keyboard smaller (in portrait mode at least) than its Evo 4g counterpart. Thankfully it is swype so it doesn't matter too much, but for the words I had to type, I was definitely making more mistakes than normal.
MAIL:
There were a number of improvements to the Mail app from the Evo 4g
1. Portrait Body Preview: which adds up to 5 lines of the body of the email to the list of emails in portrait mode. I have mine set to 2. Nice feature
2. Compose icon is in Email view, not hidden in menu
3. Unified Mailbox: As with the EVO 4g, I can switch from different email accounts set up in the Mail app, however I can now view all accounts in one list, with each account color coded. I like having the option, but I prefer to have my exchange and hotmail separate, and I use the gmail app for my gmail.
4. Email list has a 'group action' column.
5. Specify other folders apart from Inbox to sync: I like this because my boss's emails go to a specific folder and I can now schedule sync that folder.
CALENDAR
The calendar is pretty much the same as the Evo 4g. One difference I noticed in the 'Agenda' widget is, on the Evo 3d, it defaults to showing you your agenda starting today.
On the Evo 4g, it shows your agenda starting today AND at the current time. So if I had a meeting at 12 noon, and it is 2pm, this meeting will show up on my Evo 3d agenda widget even though it is in the past, but not on my Evo 4g.
That's a step back IMO.
People Widget
The people widget can now be reorganized however you want. This is great because my boyfriend's name starts with an X and was always at the bottom on the Evo 4g which sorted only alphabetically.
Other Features
You can block a contact from calling you. Nice feature, not sure I'll use it.
3d camera: the 3d photos that came with it are gorgeous but I have a point and shoot that I love and the Evo is really only for emergency photos and I don't need 3d for that. I also don't have a 3d tv or any other medium to view the photos except the phone. Do not need.
360 home screens: Nice feature but unimportant to me
It also takes forever to charge.
I haven't activated it as I just got it to test it out and see if I really want to keep it. So I can't comment on voice signal, but I live in Austin TX and I have had ZERO problems with signal with Sprint on several phones I have had with them so I am not bothered about that.
It's a very nice phone and it impressed me. However I will probably return it. I bought it full price so I am still eligible for my upgrade. The Samsung Within will probably be the phone I use that on.