I've been playing with the colourise and other filters, and they are fun, unfortunately they show up the shortcomings on my creaky old system
One thing I have discovered is that it pays to spend some time getting brightness and contrast better before applying the filter. In the context of the filter, "better" often means more marked rather than better to the eye, as ever the histogram is your friend. Probably more significant on scanned b&w snapshots rather than negs. The AI works best when it "knows" some of the colour, for example a sandy beach, but it isn't beyond making some of it green, coz after all ground is grass is green. A good tutorial is to find a photo you know the colours in, eg a school uniform, and play with the curve until it interprets it as the right colour. The tweaking controls can be used to improve the final result, but they can't change, for example, red to blue.
Another use for the colourising tool is re-working faded colour snapshots, when it really smashes it out the park.
I haven't been playing with the more esoteric filters yet, but another useful tweak to know about is the half tone screen filter (in photo repair tweak controls) that does an interesting job on newspaper photos, use with care if you don't want a boy's own comic look.