I fitted a set to my Volvo 940, and had a few problems.  Can't remember the
brand but it wasn't the ones you mentioned, however my experience is
relevent I think.

1. Accuracy is poor and I think less than specified in my case, but will do
what you want and protect your tyres.
2. Mine came with milled/drilled aluminium valves that the sensor bolted
onto inside the wheel with another bolt that clamped the valve and seal into
the wheel.
3. The aluminium valve stems didn't seal and one broke off during fitting,
another broke off when I put on a garage tyre pressure hose, leaving me with
a flat in 30s.
4. The company promptly sent out replacement steel valves that sit in more
like conventional valves, these worked better are are in now, however I do
worry that they aren't replaced like normal valves.  I guess they will last
as long as the batteries (5 years+).
5. I positioned the receiver in one corner of the windscreen and the
farthest away sensor isn't always picked up in the long estate car.  They
seem to transmit occasionally and when the pressure changes.  The receiver
is always on and uses little current in standby, so it warns you when you
put on the ignition if the tyre has gone down earlier.
6. The aftermarket ones are a little more flexible than the OE ones, that
assumed a high level of accuracy and the warning level is programmable.
7. Most tyre places now will tackle the job, but usual I seem to end up the
17 year old that doesn't know how to get the tyre off without damaging the
sensor and needs supervised.
8. I left the car in a good neighbour in London and little bastards stole
the chrome covers that came with the valves.  Hopefully they won't feel the
need to take the black plastic ones I replaced them with.

I would advise a system that wraps around the rim rather than bolts to the
tyre valve as this should be left alone.