As a one-time user of Eurostar (so far) I have a few observations about the service:
Firstly, the checks at St. Pancras on departure seemed very much like "security theatre" rather than a genuine check. A lack of signposting and information made it difficult for a first-time user, as there seemed to be an assumption that everyone should already know the process. As someone on the Autistic Spectrum this was a bit stressful for me, but I could perhaps have arranged support with Euro* customer service had I planned ahead better. The main issue was the behaviour of other passengers at the baggage scan, and better staffing levels would have helped here. The exit to the baggage scan was like a Black Friday scrum, particularly upon departure from Brussels on the return journey. A particularly rude English woman caused me to have a near-meltdown due to her impatience as well as tearing a brand-new rucksack by barging me and said rucksack out of the way. Better supervision of this process would hopefully prevent such petulant behaviour from the self-entitled. At least on my next trip, I'll know what to expect!
I booked about 15 days before travel, and the cost for my entire journey (Huddersfield to Prague) was cheaper than any airline fare I could find when factoring in the cost of getting to the Airport in the UK. I don't recall the exact fare without looking it up, but a CIV ticket (not hidden at all on the Euro* website, contrary to what others have suggested) for the Leeds to Brussels-Midi leg was around £80 return. The Brussels-Midi to Prague-HN leg (admittedly via a slower route to save money) was a total of £40 return booked on the CĎ website. Sure, I also had an outbound overnight stop in Brussels costing £25, and spent a total of 24hrs in transit on the return, but it was an adventure- air travel is something I find dull and stressful without any of the positives you get on the train, but I guess that's just my opinion!
I do wonder what the future holds for the Brussels route post-Brexit, it may well be that Amsterdam becomes more popular, with Brussels services being diverted there if/when the security infrastructure is in place. If service to Brussels is reduced, this might make longer-distance travel to Germany and Central Europe more difficult by rail, though it'll already be a marginal market anyway.