I've run X2, X4 and X5 with mixed levels of success using 5E rules.
X2 took a long time to convert, but hell was it worth it - my players loved it, with its wacky pseudo-french theme. It's very very long though, as battles tend to take a little longer to resolve in 5E than they did in B/X. Averoigne could do with a little more fleshing out.
X4 was also a hit, the set piece nature of the events worked well with 5E. It's as far for the 6-8 encounter day as can be imaginable, but it still works, due to the big battles being very big. Thus it was no problem for with the PCs going all out with everything they had. The Abbey at the end is brilliant.
X5 was a bit more troublesome, it took me a long time to decide what I wanted The Master to be. It all ended with near TPK, my players enjoyed it, but it was hard work behind the scenes for me.
Had they been successful in X5 we would have continued to X10 - now THAT would have been a challenge!
I'd love to give B4 a go using 5E rules, that's my favourite in the B series.
Sounds like my party. They went north to Glantri instead of south to the Isle of Dread so I ran X2. I tried to keep the flavor of the module as much as possible. I kept most of the save or suck situations. I had been running Gritty Realism so the method of regaining HP/resources in X2 was welcome. Especially after I altered from "when the session ends" to getting HP/resources back when you left an area of the Chateau so we had the silliness of pretending the session was over (wink, wink) and then immediately welcoming the players back seconds later.
X2 went shockingly well after a series of modules by the UK team (U1, B10, UK2, UK3) that are quite realistic in theme and setting. I set them up with a bit of background from a YouTube video and the warning "no logic was harmed in the making of this module". I knew they had bought into things when, in the first encounter in the boxing match, I asked them in my worst French accent if they wanted to box and a PC replied, "yes". I responded, "Your accent! It is so strange!" and, without missing a beat, "Oui".
The possession by the mad Princess Catherine was a major hit. I picked the PC most likely to be able to pull it off, gave her a write-up and said "this happens when you're ready." She had been previously turned to stone so she didn't have much to do. Stone to Flesh is a 6th level spell and, since I had Catherine as an 11th level Wizard, I pretended she had used the spell to turn her back to flesh and then possessed her. She stayed possessed and doing a horrible French accent until almost the very end where we almost had a TPK right before the PCs returned from Averoigne to the Chateau to free Etienne. The player of Catherine correctly reasoned she wouldn't want to actually free her husband. It was so much fun with the almost-TPK I skipped the series of encounters after returning to the Chateau and had them free Etienne forthwith.
X2 was more fun than I could have imagined. Play up the crazy and have fun!
Then they used their broken time machine (Think Back to the Future. One of the PCs is a Glantrian Sorcerer who can channel the Radiance, which is powered by a magical nuclear reactor. Given the hint of the lame Back to the Future ripoff he reasoned he could use his sorcery abilities to power the time machine) and a random roll sent them to X4, which I've set 63 years before the campaign date.
We even played a game of Parcheesi (one of the random encounters in the module is peasants. I made it not-random and the first encounter) to get the faux-India vibe. We got as far as them agreeing to the mission last session. I think Gritty Realism will work well with X4.
BTW, what
did you decide for the Master to be? I have him as basically what he is in the module, a high level cleric.
