My first (ever!) KL flights recently and I have to say my eyes have very much been opened! Only a short hop over to AMS & back, also on Cityhopper as opposed to their regular mainline. However, unless I'm mistaken the only difference being the plane rather than what you'd get in terms of different services between BA & Cityflyer.

Good
It's still by no means the best airline app, however... absolutely blown away by just how diabolical BA's entire IT offering is.
Check-in was flawless, no issues. Being able to select economy comfort, or whatever the extra legroom seats was a nice touch. Automatic 'live activities' (Apple) keeping me informed throughout of flight status, displaying the gates at both AMS and outstations. Pro active notifications about a minor delay (15 mins, due to late arriving crew). Points were posted the same day for the flights taken.

Complimentary F&B onboard, nothing to write home about but refreshing to have a choice of hot & cold drinks and given a sandwich.

The Lounge as an actual space was great. A huge variety of seating types, zones and places to either relax or work. I haven't been in Galleries Club for many years, so maybe not the most fair comparison. Food wise, a lot less variety to select (Crown Lounge 52), but the quality was 10x better than the at times lowest bidder outsourced canteen food GF has been putting out recently. Nice to see a staffed barista bar as well, not just machines only. Less soft drinks variety, dispensers only compared to mini-can city with BA. The bathrooms, 100000x better than the hospital/prison cubicles that BA provide. No need for a shower so can't compare yet.
One of the main things that struck me in the lounge, I didn't see a single advert, product placement or any square inch being sold to the highest bidder for a fraction more revenue.

Bad
Coming from the UK and feeding in via Cityhopper, I guess I better get used to infinite bus gates at Schipol both in & out. Didn't seem to do a BA and be 'surprised' at a flights arrival and struggle to find more than 1 bus to serve a flight.

No First Wing or compactness of Terminal 5. Can't be too harsh on this one as TFW a) wouldn't have access to thanks to the recent Exec Club changes and b) KLM don't offer a First product. It's quite a trek through Schipol to the Crown Lounge and then back down to the D gates, so need to adjust my times. I certainly don't feel as comfortable arriving as late as I could do, with the uncertainty of security queues and more importantly immigration.

Flight frequencies, I've got used to BA being pretty good in terms of frequencies to most destinations. A smaller fleet and having to connect & change has drastically cut down my options for day and short trips.

Flight upgrades not eligible for higher points & XP earning rates.

Mentioned above, the F&B variety was a lot less.