OuiGo fares are cheap, but they have 3-abreast seating and you have to pay extra for luggage or even to have a plug socket at your seat.
I am no particular fan of Ouigo's shoehorning of low cost airline practices into rail, I think the London Midland (continued by WMT)/Chiltern model of offering low walk-up fares on a slower train that's running anyway is a better one in many ways, though that may just be down to what appeals to me personally. The luggage fee is particularly silly, I don't overly care about sockets because I own a power bank and my laptop has 8 hours' battery life (as did my previous cheapo 300 quid HP, so it's not hard to get one that does - and that one had a swappable battery pack so could be kept that way at very low cost).
However I do feel I need to defend them here with regard to the seating.
The seats fitted to their trains are the Compin (?) type that are used in most older Electrostars, and in First Class on 350s and 450s, i.e. these:
Interior of Southeastern Electrostar, from showmethejourney.com
They aren't my favourite, but they certainly aren't bad and I'd happily sit in one for a many-hour journey. They are hardly ironing boards, and a complaint about them is very rarely heard on here - the only one you do seem to get is the armrests being a bit high for some (but you can always put them up).
The 3-abreast thing also requires a bit of analysis. The layout with 3-abreast only exists in part of the train, and is because of the First Class areas being converted to Standard, and the presence of a sunken aisle in First Class on the double deck TGVs so they can't do 2+2. The 3-abreast seating is in a 3
+1 layout, not 3+2. This actually has some strengths, not least the bays of 6 being good for families/groups and the individual seats being great for, well, individuals, plus it feeling less cramped with 3 people across than 2+2 does, and I've proposed some TOCs should consider doing this anyway even when not forced because people may sometimes like it!
So will HS2 have this layout? Almost certainly not, particularly given that the published HS2 train spec specifies 2+2 in Standard.
So I think the only thing we're really needing to look at with Ouigo is the fares structure, which if you take away the silly charge for luggage is just a yield-managed Advance-only-type structure, and one I think HS2 is near-certain to have anyway, i.e. basically as per Eurostar. With regard to seat selection I don't overly mind paying a little for it because to get optimal use of seats with compulsory reservations you need at least some people not to select, so e.g. if you've got a train doing A-B-C-D you can put A-B, B-C and C-D reservations in the same seat when people aren't bothered about where they sit. If everyone selects, you get a sub-optimal allocation and so end up being able to sell fewer tickets because the train is "full" even though practically it isn't.