Maybe I am a little late to the party, but here are my experiences:

I got a very early ST70 head from Defender, before Raymarine said they were available. Discovered it didn't get along well with the 3G computer, so sold that on ebay and got a very early X30 course computer. Had issues with the course computer not reading the flux gate right, was replaced by Raymarine with newer version of hardware. Now about 2000 miles or so on them.

The autopilot head is very nice, neat graphic display that tells you all you need to know. I agree with the criticism about the button presses needed to dim the display and would add two others: to get to the sail-by-wind mode you must go into the menus, one press right I think, then enter, then cancel to get out. This is in contrast to an ST6000+ which required only simultaneous press of -10 and +10. Also, it always powers up at response rate 5, when under sail 1 is usually fine except maybe downwind in boisterous conditions. Must enter menus, three presses to the right, set to 1, then enter, then cancel to leave. Other than that, good effort by Raymarine - I would not trade it for the ST6000 or other previous versions.

The X30 on my boat is driving a very powerful Jefa geared servo. The course keeping is pretty much as good as you want it to be, steered for 22 hours across Lake Huron DDW in a bit of a nasty seaway, never a worry. The gyro pilots are much better than the non-gyro downwind, I think the X30 is slightly better than the 3G that preceded it. I can still steer a little better course myself, but only for about 15 minutes. Contrast to the older ST6000+ on my last boat, downwind in quartering sea a little overpressed and it would lose control and give up within minutes.

I'm not sure about the fast rate heading not supporting Marpa on the E series. Mine seems to work, don't know what I am missing maybe? Have to load the 5.52 software maybe? I have the X30 and E series connected with SeaTalkNG (NMEA 2000).