The UX (both keyboard and software) was .. just awful.
The software + hardware was really poor, even for the time. (Even the original Raspberry Pi is significantly faster -- I know this isn't a fair comparison, but just for reference)
Also a lot of the OLPC functionality didn't work reliably, or didn't work at all. I bought it specifically for the mesh networking, which was cut entirely. The devices did work over WiFi, but would struggle to see other OLPCs over wifi reliably, even when connected to the same AP. I spent a lot of time reading forums online to troubleshoot and download fixed drivers and such. I remember explicitly wondering "how are children in poor areas who depend on mesh networked internet, supposed to be figuring all of this out?".
The screen was hard to use. The e-Paper mode was nice, but the regular mode was blurry/grainy and slow and dim and just difficult to see. In terms of Software + UI + responsiveness, it felt more like a really big Palm Pilot, and less like a laptop.
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I loved the mission, I loved the ideas, I loved the design. The exterior of the case really was durable and kid-friendly. I loved the idea of the screen. I threw them more money than was reasonable. I was just fairly underwhelmed by the device as it actually shipped. It felt like a prototype of a dev kit, which is fine. But it was sold as something for children to use, and at least at the time I messed with them, it was nowhere near ready for that.
But I remember being most disappointed about the mesh networking, since that was supposed to be the thing that made the project feasible. I intentionally bought two of these devices just so they could talk to each other -- and they couldn't reliably do so, even with conventional WiFi at their disposal).