Overall I have very much enjoyed the new editor so far. I think it's a great addition to the game and is a good way to get the community involved in content creation, as well as simplifying the basic process for existing mappers who, like me, might lose their ideas and/or motivation quickly in Hammer because it can take hours to get something that can even be compiled and played in-game, let alone a finished puzzle.
Like:
It's very easy to lay out a puzzle quickly, and I can get an idea from my head into a working chamber in minutes rather than hours. I also find that the simpler editor makes emergent design a lot easier, which is good because I don't have a lot of existing ideas and kind of need to make it up as I go. 
I still really, really like the fact that a faith plate's trajectory is drawn on-screen and can be adjusted with a click of the mouse.
Dislike:
As has been said before, it's hard to judge exactly where a fling will send a player. I spent about 30 minutes yesterday tweaking a propulsion-gel-aided vertical fling in the hopes of getting it to send me to a certain height so I could land on a platform, and I ended up having to make a different solution because it simply wasn't possible with the rest of my layout.
My other concerns/complaints have already been mentioned: no regular doors between sub-tests, 128x128 is sometimes overly restrictive, etc.
What needs to be fixed:
Honestly I don't think there's a whole lot I would say needs to be "fixed" - my gripes with the editor are more from its design than anything that seems outright broken. The two things that I would say ought to be fixed, however, are that I occasionally get stuck in a position where I can't adjust my view with the mouse for some reason, and clicking and dragging things from the test-elements window doesn't always work on the first try. Objects will sometimes get "stuck" to my cursor as well when I try to place them in the level.
Anything about the editor that annoys you:
See above.
How do you feel about other's maps:
I like the maps I've played from others, and I like that I've seen people I'm familiar with in other online communities designing their own levels. Granted Sturgeon's Law definitely applies to the workshop as a whole, but with 32,000+ maps, if 10% of those maps really are worthwhile, that's 3,200 worthwhile test chambers!
Will you continue to use Hammer, PeTI, or both:
For now I'm content exploring the possibilities of the in-game editor, but I will definitely get back to Hammer eventually, especially once the in-game editor gains co-op support. For now I just want to design working puzzles; for now my only intentions for Hammer once I go back to it are adding visual appeal and possibly optimization.