You asked, "Is it a good alternative to dedicated power meters with very expensive external sensors?"
It was for me, because as you said, the power meter sensors are quite pricey. The power measurement in the EIP 575/8 pretty much works over the range of the basic counter - from about -30 dBm to +10 dBm. So it may not be quite as flexible as a dedicated power meter. But at least against my various signal generators, it seems quite accurate (well less than 1 dB).
I forgot to mention in my previous post, that these are "source locking" counters. When I first got mine, I thought, "what is source locking and why would I ever need it?" Turns out to actually be quite useful. The basic idea is if you have, for instance, a cheap function generator (or synthesizer) with an external VCO port. You can feed the EIP (back panel output) into the VCO input, and then feed a tap of the generator's output back to the EIP input. Punch in the frequency you want on the EIP and the EIP will attempt to adjust the VCO until the generator locks to your desired number. It seems like magic, but really works well. With your EIP unit with the OCXO, you now have frequency stability for your cheap generator. There is a limitation on the frequency resolution that you can enter into the EIP (something like 10 kHz if I remember correctly), but if you are trying to externally tune an oscillator via a DC level into its VCO port, it sure beats hooking up a DC supply and hoping it doesn't drift much.