Juniper VPN compatibility with other VPNs?
Juniper Netscreen SSG-5s use standard IPSec. As long as you have an IPSec VPN software client you should be able to connect to the SSG. Keep in mind however that software IPSec VPN clients are sometimes flaky if they are free or you have to buy one to pay for the stability.
You may run into could compatibility problems depending on the software client and how the SSG is configured. Trying to troubleshoot this from the software client side can usually be difficult depending on the level of verbosity in the debug output of your client as opposed to an SSG firewall.