Yes, there are currently 4,533,948 seti@home users including several on this forum.
For Windows systems (95/98/NT) you'll need a computer with at least 32 MB of RAM, the ability to display 8-bit graphics in 800x600 resolution, 10 MB of disk space, and an Internet connection (dialup is OK). For Macintosh systems you'll need the above, as well as a PowerPC processor and Mac OS 7.5.5 or later. You can use SETI@home on a laptop that is connected sporadically. We also support many versions of UNIX, including Linux on various CPUs. We hope to eventually support other systems such as OS/2 and BeOS. We won't support WebTV. There are no CPU speed or modem speed requirements. It doesn't matter where in the world you live.
Yes. Of course if you have an always-on internet connection, you should take the proper precautions, but the seti@home software doesn't connect unless it needs to send/receive data and you can set it to ask you to connect when it needs to, or automatically.