I'm using the RSI Pressure Trace gear, and it seems to work pretty well based on what I've done so far. It is a strain gage system that ties into your PC for the computations. I have gages on my 336 in 35 Rem, and my 1895. I specifically bought because the 35 was performing way under book velocities, and I wanted to have some backup as I started to test +P loadings. Based on my testing so far, the 35 was giving much lower pressures that loading manuals would indicate for specific loads, and as a result much lower velocity. Right now, it takes 2+ additional gr of powder just to match book pressures - a really interesting outcome. By the same token, the 1895 is pretty close with expected velocity and pressure with any given load, also interesting. I really like a couple of features - you see the full trace of pressure from ignition to exit, and the program overlays the shots so you can see any weird behavior; and it also provides Optimum Barrel Time (OBT) theory calculations that are supposed to show "best" accuracy points, which actually seem to work in both my rifles. It allows you to change load velocity to try an match best potential accuracy points just by reading the chart (you have to input chronograph data manually for each shot for it to work).
Support for the gear is good - I had an early unit, and current production has some design changes. I was able to get the upgrades installed cheerfully and at no charge - a welcome change from the norm.
They are a pain to set up at the range - I try and make the most of every shot, so I'm shooting for velocity, accuracy and pressure all at once, and that's a lot going on. Also, my experience so far is the gages are heat sensitive, and you have to wait quite a while between groups, or your data gets hosed up.
Unit reads directly in PSI, and it is critical that chamber measurements are accurate for the system to work well. Using factory ammo can provide a "standard", but its really not a calibration per se. Many loading pubs still show pressure data in CUP, so you have to understand the difference - the conversion is non-linear. That's not too much of a problem, as most rifles are of known safe strength in PSI and you can work accordingly.
I haven't tried any other units, so they may be just as good. Its only an opinion sample of 1.