My friend travelled from Portsmouth to Brighton on Saturday, then we both travelled together to Portsmouth with myself heading home on the Sunday.
Our 1h 45m journey from Brighton became about 3h 40m, travelling up to Gatwick, then on GWR to Guildford, then on SWR down to Portsmouth because of this (and GTR only arranged acceptance with GWR after I DM'd them on Twitter to ask, since this brought the journey time down by about 30 minutes).
On the Sunday, to avoid rail replacement on SWR, I ended up going from Portsmouth to Eastleigh, then up to Clapham Junction and down to my home station, departing at 14:22 and not getting to my destination until 17:19...
Really annoying, and I heard some SWR gossiping about how GTR (supposedly) just asked Go-Ahead buses (e.g., Metrobus/Brighton & Hove) who said no.