Been using AS5 for a week or so now. Here are some things I've come to dislike:
1) The editor is slow intermittently. I can easily out-type it sometimes, leading to a very unpleasant experience when you feel that you typed wrong, do a few backspaces and then look up..
2) It takes 80 seconds to start.
3) Stepping with the Dragon in debugWire mode is extremely slow. Example: This line
if ((SWITCHES_PIN & (1<<SWITCH_PIN_NUMBER)) != previousState)
takes about 7 seconds to execute (step over). Stepping a "really trivial" line of code (eg something that should generate more or less a single instruction, takes a second or so! That is just plain ridicolous. I can not recall that doing debugWire in AS4 was that sluggish.
4) In the AVR Programming dialogue, in the Fuses page, in the combo box for SUT_CKSEL - instead of a proper readout of the options as in AS4 there are some gobledygook that I will have to turn to the data sheet to decode. And the combo-box closed is not wide enough to show a complete selection (where everything is more or less significant) while there is ample space in the dialogue for it to be wide enough (that is just sloppy UI design. Designing UIs should be left to UI experts, not application programmers).
5) Try resizing the whole AS5 window. Extremely sluggish - I'm talking seconds to react, resize and repaint.
6) Perhaps not AS5-related: I just experienced one hour of hell when I tried to get my ATmega88 (actually sitting on an STK-500, but the STK-500 ISP header attached to a Dragon) out of debugWire. I know of the mess with having to remove the STK-500 Reset strap for debugWire, but needing it in place for ISP. And I know how to get out of debugWire the proper way (by selecting to disable it in the Debug menu). It just wouldn't bite. I needed to get out of debugWire to look at the clock selection fuses, but for two hours I failed repeatedly. Then suddenly, when disabling debugWire, I got four or five cycles of USB-disconnect and -connect sounds, and it suddenly worked. It is truly sad that there is no way to handle fuses in debugWire.
7) There is no shortcut to the AVR Programming dialogue, and when you get to it you have to Apply the Tool selection every time. In AS4 the programing dialogue could be left open, so after re-building it was a one-click job to program the device. In this respect AS5 is "a regression".
One thing I like: The ability to type in a filter for the device selection listing for a project.
One other thing I like: AnkhSVN works in AS5. :)