Luckily, i have a VERY stable line, so don't get many (any) problems, really.
More speed is available to me via TT if i wanted it. I know some people on TT and they're more than pleased with how things worked out.
But i don't really need the extra speed.
I don't need to use their mail servers much so haven't seen the problems you had.
During daylight hours they seem to be ok for me.
Also, 1899 still works ok.
One thing that has slightly deteriorated for me is 'ping'.
The first hop is still the same as ever, but around 10ms or so seems to have been added by the BT network at Ilford after arriving from 'up north'.
Now my 12 months is almost up i could consider moving on, but will stay for now.
Good Luck with your new provider, Plusnet.