I don't use any commercial VPNs (we have our own setups based on OpenVPN) but I can think of a few things to be aware of:
(I'm assuming you are looking for an external service, rather than a VPN server installed/configured at one of your own locations, as that is what your question seems to suggest)
- Check the small print very carefully on anything that is labelled "unlimited". Anything that is even remotely close to what the word unlimited means in CommonSenseLand will be vastly oversold so you may experience significant bandwidth contention and latency.
- If this is for business use (or personal use where the VPN not performing will cause significant hassle) check for uptime, bandwidth and latency guarantees. The latter two will always be pretty fuzzy as there are many factors beyond the provider's control but the former should be more definite and backed up by historical figures.
- Be aware that you could experience double latency between two of your end-points as there is the packet travel time between site 1 and the provider then the provider and site 2.
Some VPN solutions try to use direct connections where possible including employing STUN-like methods (see here for details on that) to make this work through some NAT arrangements - but this does not work in all cases. When it does work the extra latency is removed (packets go directly between your end-points, not via their hosts) and they can truly offer unlimited bandwidth as all the bandwidth you use of theirs is during the connection negotiation.
- Make sure the provider has services hosted near your indented end-points. If you are in the UK and connect two end-points to a US-only provider, and STUN is not used or does not work with your NAT configuration, you will end up with round-trip times between your end-points being 200ms or more higher than they could be with a more local provider.
