I've used a poncho in the UK, and it worked quite well when walking on the flat, even in gusting wind and driving rain.
However, I discovered that, for me, it was a problem when ascending, and even more so when descending rough terrain in bad weather.
Gusts of wind tended to make the poncho flap around and obscure your feet. It was unsettling enough for me to think - this isn't going to work!!
For mostly flat terrain, I think they can be quite good, but I came to the conclusion there were better options for the walking I do. Namely, a short jacket.
I think Roger Caffin has described his design of poncho which seems (from what I remember, so I could be wrong) more like a short jacket with room for a rucksack. Might be worth checking out?
With pack covers, I don't bother any more. When walking through cloud, everything seemed to get soaked anyway, regardless. I've also had a pack cover ripped away by the wind - it actually wrapped itself around my head - I was pretty confused for a couple of seconds I have to admit, alone there in the sudden darkness.
Now I use a set of different size and colour Exped dry bags to sort my dry gear. A tiny orange one for personal effects, a small yellow one for my down quilt, and a large red one for a belay jacket, gloves, hat and whatever. I haven't used my XL blue one yet - maybe next winter. It makes finding things much easier and quicker. They weight about 50g each, on average. I did try a single large plastic dry-sack, and it was lighter, obviously, but I found it less convenient.
A pack cover did seem like a good idea, but in the end I found it was extra weight that didn't actually provide sufficient protection in the wet UK, and which also made it harder to access stuff in the rucksack when you wanted to.
I'd probably take one for casual day-walks with the kids, as the Exped bags wouldn't cope with all the clothes they hand me 1/2 a mile from the start :o)
Hope this helps a bit.