Oli4

I took a chance and bought a 200Ah one.
The Wh number on the comx site is wrong  when you do the math and you can see that someone edited a 180Ah manual to indicate 200Ah - which makes it seem very dodgy - BUT the included manual that I received have numbers that make sense. (it seems that there are 180Ah and 200Ah units. 180Ah was out of stock with mustek/mecer when I enquired)
Overall outer condition seems fine (the battery shell is plastic, but no obvious dents/cracks/scratches - shell actually looks 100% - no idea what the inside looks like and will not take to opening it up...yet).
battery terminals had no sign of use/tool marks etc.
Specs indicate it weighs "approx. 23Kg" - mine weighs 23.4Kg (google suggest the specific energy of LiFePO4 to be 90-120Wh/Kg. So while not knowing the exact weight of the cells, the numbers should get you at least past 2000Wh (160Ah for 12.8V nominal)  at the lower spectrum ...I guess it could contain only 4 cells and some lead bars...)
My cheap multimeter showed a voltage of 13.19V  before hooking it up for the first time (probably fairly meaningless for a lithium, but at least it did not arrive @ 10V) 
I charged it with a Multiplus inverter/charger @ 35A (the manual indicates standard charge as 15A and maximum as 100A) - I was keen to have it charged as I knew we were in for load shedding later the day. It took close to exactly 3 hours before indicating "absorption". Multiplus held absorption for another 1 hour (current tappering off). By my math (which is worse than my spelling) using info from my VRM log, by the time absorption was reached the battery "accepted" about 105Ah and absorption probably added another 15Ah. As far as I know lithium batteries are shipped at about 50% charge. So assuming the shipping charge state and the known charge cycle the math would suggest that it probably is in the 200Ah range.
While charging nothing felt warm (my hand being the closest to a scientific instrument for temp testing).
When load shedding takes a break I  might see if I can get a more definitive answer on the capacity with a sustained fixed load.
The battery has been used this past week and seemed to happily power the very modest average 200W load for 2 hours, then charge @ 15A and be ready at least an hour before the next load shedding session commenced, and repeated this cycle another 2 times per day for 4 days in a row. (I lowered the charge rate to 15A to "baby" the battery when possible, but if we hit stage 6 load shedding I will up the charge rate again). 
The battery has done momentary (couple of seconds) discharges @ >250A (NOT a typo) and so far nothing has melted/tripped/caught fire. The manual indicates maximum continuous discharge as 100A - no information on pulse current. How well the BMS does what it is supposed to do (or whether it is even there...?)  I do not know  and have no intention to short out the terminals, or charge it to 16V to find out. 
The "manual" is 1 piece of A4 paper, but at least it is double sided, glossy, in colour and not written in the worst dialect of Changelese. Probably the biggest issue is one part of the page indicates a charge voltage of 14.6V, and another 14.1V - I suspect the 14.6V is probably the high cut-off. (I set the voltage at 14.1V)
Whether the battery will give me 1500 cycles (this I assume is to another 80% capacity loss) I do not know.
Should you consider one, or more, of these keep in mind (according to the manual):
You  can NOT parallel connect to get 400Ah, but you can series connect to get 24/48  "Bulk"/absorption voltage is 14.1V (float @ 13.6V) for 12V system "Bulk"/absorption voltage is 28.2V (float @ 27.2) for 24V system "Bulk"/absorption voltage is 56.4V (float @ 54.4) for 48V system  

I'm not sure if the following is legal in terms of keeping municipal and rainwater/greywater separate but it's something I'd do without it bothering my conscience.
If you want to make the change over system automatic you could install a ball float valve (like the ones in your toilet cistern) inside and close to the bottom of your rainwater tank and connect it to the municipal supply. When the rain water level in the tank drops to say the 25% capacity the ball float valve will open and maintain the level at 25% using municipal water. Just determine where the crossover point needs to be by moving the ball float valve up or down inside the tank.
Your booster pump will never run dry unless there is no rain water and the municipal supply is off and you'll never need to worry about manual intervention. Alternatively install a float switch in the tank so that the booster pump can't run when the tank is below a certain level.
The ball float valve prevents rain water from feeding back into the municipal supply although the municipal pressure will almost always be higher than the head of water in the tank (except when there are water outages or if the tank is on a very high tank stand in which case you wouldn't need a booster pump). Regardless it's not possible to accidentally feed rainwater back into the municipal supply using a decent float ball valve.
N.B. Just make sure that the outlet from the ball float valve is in clear air (not underwater) when the valve starts to open. This prevents siphoning/feedback into the municipal supply when the municipal supply is off and the level in your tank is in the "top up" area. You can do this by bending the float arm downwards or installing the valve at a tilted angle so the float is lower down than usual.

Hi NickM,
I am not an expert, but I did my rainwater setup 3 years ago - first connecting the toilets and washing machine, then connecting the entire house once the water restrictions kicked in in Cape Town. I did some research at the time.
Rainwater can indeed be quite acidic. I measured the pH a few times and it varied from 5.5 to 7.0. So if your copper pipes are long, or difficult to access, you are probably exposing yourself to some annoying leaks after a few years. My best experience is with Cobra pipe. My worst is with PVC pipes (leaks around the glued fittings if the pipes move a bit). In the setup you describe, I am a bit worried that you are using connected piping for potable water and non-potable water. Rainwater is not "clean": the moment you store it in a tank, you will have germs, algae and bloodworms. Running that water through the pipes that are also connected to your potable water at other times of the year could be problematic, from a health point of view and from a regulatory one. In my case, I did a parallel setup with a switch by each toilet (what you described as "that would be madness" 😉) - but I have enough water to never have to use those switches. Hope this helps!