So I ran some tests. I got all quantitative and sniffed the packets to get precise timings and such. I used this page
for testing, as it has many small graphics (suitable for pipelining). And I set "network.http.pipelining.maxrequests" to 8, because I thought that sounded better than 4.
Without pipelining, it takes maybe 8 seconds for just the network activity and uses about 134 packets (both ways). With pipelining, it goes down to 5 seconds and uses around 124 packets.
Conclusion? It *is* faster, but in all honesty, it will have no impact on my life whatsoever. It's like overclocking your processor by 1% -- you feel good about it, but it effectively changes nothing.
It probably has a more noticeable effect on dial-up, though. And at the very least, you can taunt all of your friends about their lame browsers that don't use HTTP pipelining.