Most of what I have to say has probably already been said above, but i'm tossing my two cents in anyway. Long story short: I love the new stuff, but it might need some time to grow on you.
Personally, I'm a fan of the idea of the new software. It fixes most of the complaints I've had of Simatic Manager. A lot of them are simple things: being able to save your ladder code even if the block has an error, being able to do indirect addressing in ladder (with the new CPU's only), or being to copy/paste from a DB to excel. I especially like the new HW/network editor. It cleans up safety programming a bit as well. As a whole, the software shows really well to a newbie, but it can a bit of a shock for someone who already knows what they are doing.
It definitely still has its gotchas, though. V11 and V12 were pretty buggy, but the most current version (V13 SP1) is a lot better. A lot of the program works differently while you are online with anything (say for monitoring code): you can't edit the HW config of any device in the project if you are online with even one of them. Unfortunately, some of the nice features are for the newer processors only; if you use S7 V13 for the 300's or 400's, you get to use the new shiny GUI, but the CPU only has the same features it had before. It also doesn't support multi-user environments as well as SImatic Manager
The things people said about needing a newish PC are probably true. Get plenty of ram, a high res screen, and DEFINTELY get an SSD. Portal only runs as one process, so the jump from 2 cores to 4 cores won't add much.
In my opinion, the transition from WinCC Flex (and multipanels) to WInCC V13 (and comfort panels) is pretty small. The buttons are almost entirely in the same place, and the project migration is pretty straightforward. The main gotcha is the switch from 4:3 to 16:9 resolutions, so you might need to adjust your graphics.
On the PLC side, the transition is less smooth, especially if you are a power programmer. The buttons are all shuffled around in the editors. If you are also migrating from a 300 to a 1500, a lot of the logic can stay, but most of the system calls and communications will need to be rewritten. The focus goes away from absolute addresses and to symbols/tags. Data types become very important, it is essentially a paradigm shift. It took me a while to really wrap my brain around it, but you'll have to get used to treating the data as a specific type, and not just a bunch of bits you can manipulate.
There are a few features that Portal doesn't support yet, so if you're using these you definitely need to stick with Simatic Manager for now. Portal doesn't support the 300 tecnology CPUs or 400H. Portal doesn't have P-Diag or Proagent, which has been a showstopper on a few projects I've done recently; it also doesn't have CFC yet. THe G120 drive configuration has been included, but for S120 drives you still need starter. On the WinCC side, it doesn't support custom addons anymore, although I'm hoping that will be added back eventually.