I've used them, with good results. But I generally prefer EL34s to 6L6s or their Russian equivalents.
FYI from one of my previous posts:
...the original 6L6 and 6L6G's dissipation limit was 19W. The 5881 (mil-spec 6L6GB) was 23W, and the 6L6GC 30W. The 6550 has a max dissipation rating of 40W, while it's near equivalent the KT88 is rated at 42W.
The 6pi3c is also commonly marketed as the "Sovtek 5881", which with a safe dissipation above 30W, isn't a true 5881. Here is some information I've gleaned in the past about this valve.
A friend of mine near NYC has tested these with a high-voltage supply and found that they will dissipate almost 42W before the plates start to glow. I've tested them as high as 40W (approx. 500V @ 80mA) for red plate conditions in a darkened room (when you can see even the smallest trace of red glow.....). Definately not a regular 6L6GC. It can handle rather high voltages (I've heard rumors of 600VDC not being a problem as long as idle current keeps disspation below specs...). The 5881 nomenclature for this tube is really misleading, as they perform like a slightly-higher-transconductance version of a 7581A or 7027A.... Very hard to kill. I've been using a quartet at 500VDC (with a healthy 45mA of idle current) in a musical instrument amp for at least a year with no signs of weakening. No emmision burn on the glass (even when viewed in front of a clean, white sheet of paper), no rattles, no bad sounds. They've been pumping out 35W/channel all this time...They seem to require a few volts less bias than a 6L6/7027--typically needing -44V to -48V where Fender amps state -48V to -52V on their schemo--and so, sound cold/overbiased and kinda nasty when you don't adjust them for a specific quiescent current.
So they appear to be safe for operation at around 30W to 35W, which puts them somewhere between 6L6GCs and 6550s. I've never heard of anyone using these in an 18W style amp, although a few people have been known to use 6L6s.
And here's a post from the tube guru "dotfret":
There are two basic Russian designs of 6p3s, one of which is like a 6L6 internally, and the other is an original Russian design.
It is possible to use a 6p3s of either design in any place that was designed for a 6L6, and they will usually work in place of a 5881.
Some of the Russian designed 6p3s will work in place of 6L6GC, but not all of them - you have to check them out, if they don't redplate they're fine - but when they do work they often give a cleaner sound, and less hum.
Here's a pic of a 6p3s-e that is OK in place of a 6L6GC - made by Reflector, Saratov, Oct 76 with the OTK diamond mark which designates it good for military use. The "e" suffix should mean "ruggedised envelope construction" and nothing else, but sometimes it might mean more (they could not change the specs once they were written down).
I'm using these with no problems, replacing 6L6GC.
That 6pi3s-e looks identical to the Sovtek 5881, with it's oval slots in the anode and thin wafer base.