How safe is it......

I have two assumptions here:
1. You don't care that someone is looking over your shoulder for your password, etc.
2. You don't care if there are viruses on the machine you are using

Yes, because it doesn't leave any traces. I guarantee you, unless you are using a beta.

You may want to scan the computer for viruses beforehand though.

"There is a tendency for the world to say to America, 'the big problems of the world are yours, you go and sort them out,' and then to worry when America wants to sort them out.
Tony Blair

I use KLDetector to check for keyloggers.

Just run it, then type some random text into notepad, and it will check to see whic files were altered while you were typing.

http://dewasoft.com/privacy/kldetector.htm

*EDIT* I do not know if this is portable, but it seems to be.
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Text signatures are so last week
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The soundest computer security advise I ever read said: "If it's important, don't put it in a computer".

Potential dangers in a cyber-café, just to nurture a bit your paranoia:

* keyloggers: a hacker could see every key you press
* sub-seven and alikes: a hacker could be watching your screen as you work
* virii: an e-mail worm might forward some of your e-mails or fragments of them in an attempt to look real to recipients of it's e-mails (and attach itself to them, of course). Other virii could delete part or all of your data.
* virtual RAM: stuff that won't fit in memory is moved to a certain file on the disk, that is used by Windows to emulate a larger amount of memory. After the app is closed and the USB stick gone, it might still remain there (flagged for deletion next time space is needed, but still there)
* troyans: that app that you oppenned from the local computer... I planted it there. It has just e-mailed me your user and pass, and then started the real app (just kidding).
* sniffers: hackers could be reading all your network traffic.

Then, of course, there is the already mentioned risk of shoulder browsing. And flame throwers won't help either, because they tend to exceed the working temperature range for USB pendrives.

All that said, most of those tecniques are a bit extreme and not usually used by hackers at cybercafés, but the theoretical risk still exists, even if Portable Apps makes a good job at not leaving valuable info behind.

There is always going to be some level of risk. You need to look for a balance. What is it that you are trying to protect? If it's just the MSN account you use to chat about with your friends, you can accept more risks; if it's the password to your homebanking then it's a different matter altogether.