I understand it's a standard practice to look at both these variables. Of course they can easily be spoofed. I'm curious how often can you expect these values (especially the HTTP_X_FORWARDED_FOR) to contain genuine information and not just be scrambled or have their values stripped away?
Anyone with the experience or statistics on this stuff?
Is there anything else that can be useful for the task of getting the client's IP address?
It depends on the nature of your site.
I happen to work on a bit of software where IP tracking is important, and within a field consumed by parter sites I'd guess some 20% - 40% of requests are either detectably spoofed IPs or headers blanked out, depending on the time of day and where they came from. For a site which gets organic traffic (i.e. not through partners) I'd expect a much higher ratio of good IPs.
As Kosi said, be careful what you're doing with this - IPs are in no way a reliable way to identify unique visitors.