I have been using this system for 10 years and it has worked fine.
By the skin of its teeth. The slightest disturbance could send it off-par, even something your neighbours do (depending on proximity) or (for example) a change to the mobile phone network.
Picture quality and sound are both good 'normally'.
Compared with what? What kind of TVs are you using as displays? Yes, on a 625-line CRT TV you probably wouldn't notice the difference, but compared with an HDMI connection to a half-decent 1080-line flat panel TV you wouldn't want to go back. It all depends what you're used to, and you are certainly not benefiting from the HiDef services (the analogue output from the Humax is 576-lines). A good analogue connection provides a picture quality I would describe as passable or OK, definitely not "good".
You will no doubt gather I am not very techy.
You still have not provided the detail of your setup (I'm sure we are all interested). How, for example, is your patch panel made? What have you used to adapt the screened stereo audio and video signals from SCART or phono into the twisted pairs of Cat5? Is the Cat5 shielded or unshielded, and is the shield grounded anywhere? Do you have any buffer amplifiers or impedance converters?
Transmission lines (the technical term for a cable designed to carry a high-frequency signal any distance) have a specific characteristic called "impedance". Although the conductors have minuscule resistance, the combination of inductance and the capacitance between the send and return conductors (the wires in each pair of a twisted pair, or the core and screen of a coaxial cable) creates an impedance which defines how the transmission of a signal is affected by joints in the signal path.
If you can imagine a wave passing along a trough of water, everything is fine until the wave meets a discontinuity in the trough - and then some of the wave carries on past the discontinuity and some of it bounces back. The same thing happens in transmission lines. The output from the Humax should be designed to drive a cable with a specific impedance, the input to the TV has the same impedance, and if all the cable and connectors between the two have the same impedance all will be well (apart from loss of signal due to distance, and the possibility of interfering signals if the transmission line is inadequately screened).
However, the output from the Humax is probably not matched - which does not matter if the rest of the transmission system is matched to the TV input. If there is a discontinuity though (a place where the characteristic impedances on either side of the joint do not match), some of the signal will be reflected back, and the reflected signal will be reflected forward again from another discontinuity (eg the output connector at the Humax), and when dealing with the high frequencies of a video signal (the colour sub-carrier is 4.43MHz) the reflected signals interfere with the main signal and degrade it. I don't know the impedance of the signal pairs in Cat5 off the top of my head, but maybe - just maybe - they are compatible with the impedance of the video system. Twisted pairs are what we call "balanced line" though, and ideally require a different drive from the single core of a coax cable.
If you are using modern flat-panel HD TVs with HDMI inputs, I recommend you enter the 21st century and look at distributing the HDMI signal from the Humax. There are adapters to launch HDMI into Cat5, or boosters to extend HDMI several tens of metres (I have a 15m HDMI link out of my matrix switch). If properly designed in, Cat5 is capable of very good performance in the right situation.
You still have not mentioned how you control the Humax from elsewhere - we may be able to offer you some tips on that.