The X was the machine I cut my teeth on. The problems with the sequencer are as follows:
1. It hiccups when tweaking while the sequence is playing, as mentioned. It also can't handle a lot of CC data; the same hiccups happen. However, this is not a 'sometimes' problem. The X uses its main CPU for all functionality, and its just plain too slow to handle the load.
2. When used in looping mode, it sends out a MIDI Program Change on every track assigned to MIDI out at beat 1:1 every time. That's problematic, obviously, since MIDI is a serial protocol, and the PC message takes priority over any notes being played simultaneously. That's the first downbeat of the loop... It may also cause your external module to hiccup (if its listening for PC's) because it will pause to process the PC. There's no way around it.
3. Generally speaking, using the sequencer makes the machine more crash-prone. Its gonna crash every now and then anyway, but it happens more frequently when sequencing on it. I believe the primary culprit in crashes is the SCSI, because, before I had SCSI in mine, it almost never crashed. But still, the sequencer makes it worse.
4. It lacks any form of note editing. You either have to 'scoop notes out (hold the erase button and the desired pad while the sequence is playing), or erase the whole track and do it again. Its not necessarily a 'problem' per se, and that's how I prefer to work anyway, but just know that you have to 'play' (or step sequence) a given part until you get it right.
I ended up using my workstation keyboard (at the time a Roland JV1000) to sequence my X. As a sampler its a fantastic machine. As a sequencer, not so much... There is one redeeming factor, however. The swing manipulation is really really good; you can get very in depth with quantization, more so than any other (non-DAW) sequencer I've used since. If you use it to sample and generate loops from those samples, then resample the loops (rinse and repeat, etc), its workable. The last thing I would want to do is use it as my main sequencer though.