Hi Aldo,
NCP is a VPN client that supports the native IPsec protocol in their standard client and additionally L2sec, IPsec/L2TP (but not PPTP) in their enterprise version. You do NOT need to use their own NCP server; it will work fine on a CheckPoint, Cisco ASA, Juniper NetScreen etc VPN concentrator. We tested the battery use with AES256 and IPsec compression on, VPN always on and it took about 1 to 2% extra battery comparing to running without the VPN (push email will take more). A little scenario:
During my holiday in Italy I used a Samsung i780 phone (locked to GPRS) with always on NCP VPN running. Did about 0,5 phonecall per day, did my email every day (10 minutes) and checked the weather forecast every day via internet (everything runs over the VPN btw). With 1 battery load I used the phone for 5,5 days before I had to reload!!!
True: you will have to install it on your PDA, but you will have the advantage of also having a statefull firewall and WLAN management. It also supports full management and profile lockdown (enterprise version) or local lockdown only (entry version).
However: the security modes it supports and variants theroff are almost endless, so are the configuration and tuning possibilities. In short: if you want to have real VPN power on the PDA: to my experience NCP is the only solution available.
Ciao,
Arjen