Thanks for the replies about the branding!
Had an interesting trip to London over the weekend (which is why I asked the question in the first place).
Outward coach was mainly all white with a small Flixbus green logo, and return was one of the fully green ones (which was also a lot nicer inside!).
Coach from Bristol to London was half hour late from Bristol and there were quite a few worried faces whilst we were waiting for it to turn up (presumably wondering the same as me if this coach from a relatively new company was going to turn up). Thankfully it did and we had about 30 people so it was fairly busy (that is a rough estimate based on the number before boarding so may be wrong but at the very least once onboard it looked like every pair of seats had at least one person if not two). The actual journey was a nightmare though, not Flixbuses fault (M4 around slough shut and bad London traffic) so we were about 75 minutes late in the end (so in realty about 45 mins lost on route considering the half hour late departure) - though I do wonder if we could have gone another way rather than through Slough town centre!
The return journey was a lot quieter (maybe 10 people) and we were ready to depart a good ten mins early and kept really good time (we were just ten mins late arriving into Bristol and that included a diversion up the M40 and then down the A404 to avoid the M4 closure mentioned above).
Ultimately I don't think they were any worse or better than Megabus, so based on that they will probably get my custom going forward if their prices remain cheaper!
My experience had a lot of delays in it as well. Mostly down to a lack of communication. Drivers didn't know the route fully so was following the sat nav (Which doesn't send you through bus gates understandably) and then didn't know if they could use the Mersey Tunnels and be reimbursed (meaning a nice diversions using the M56/M6, stopping at Burtonwood for a legal break and then getting into Liverpool around 2h 30 late. Return trip was much smoother (it was the same bus and driver. I had planned to have an hour in Liverpool but that didn't happen of course) but still a lengthy delay from the outbound trip. Not sure how it's all going at the minute as every Flixbus that I see in Chester goes a different way. I saw one going past the Train Station the other week. I believe Thandi are taking on the 021 via Chester shortly so we will see how they get on.
If they do that well I wouldn't bet on Stagecoach not selling up to them as they did the European operations. I'm not convinced there is room in the market for three operations, and I very much doubt they would kill established (and higher quality) NatEx.
Megbus have an opportunity to increase the network by a few hundred percent overnight if they wanted to, using local Stagecoach buses (Connection only though so for example X17 Barnsley to Meadowhall for the coaches to London. Cov to Warwick Uni on the U12 etc). They are yet to exploit some local bus services but they could do overnight and this would massively increase their reach. While yes it's using local buses, it's already done in some cases.
NatEx have the established demand which may help them out.
Flixbus will have price on their side if they go down the old Megabus route of only a full or nearly full bus makes a profit.
If each 3 went for more destinations, and focussed on some shorter distance but still intercity demand, there could be gaps in the market for 3 companies but if each one keeps going for the same few routes, I am not sure if the demand will be there. There are only so many people wanting to travel from Bristol to London by coach and even if everyone had 99p fares, there are still only so many people wanting to make that journey. Each firm has it's own sole positive point and that won't be enough. I think it will come down to who is willing to invest in more destinations/routes and provide better customer service. NX and Flix have dismal customer service. Megabus doesn't have the network nor the competitive prices (at least not competitive compared to how they were)
In other news, unreported here, Flixbus are changing services again. Tickets to/from Heathrow can now be purchased as well as timetable changes on a few of the routes and York now had an additional daily service.