I'm a regular (private) DIY'er and have been through some power tools in my time. That said, I've only owned one circular saw - embarrasingly a cheapie for a 'quick' job, but took longer trying to get the thing to cut the thinnest wood so reverted to hand-saw to save time!
With bad early experiences with portable power tools I resigned myself never to be so brash and stupid in my purchasing decisions again... until my partner gifted me a Makita battery power drill. Now I will be honest here as I had mentioned a number of highly-complimentary reviews of this and Makita tools in general. What particularly caught my eye was that Makita had taken the leap forward to Lithium Ion battery technology, promising to deliver improved power and capacity in their portable tools.
Suffice to say, quality and power of the drill was astounding and, after drilling and driving in over 1500 screws (and only a few recharges) reinforcing a large garden workshop floor, my faith in battery-operated portable appliances was restored by the Makita brand.
So, when asked what I wanted for Christmas, it was a Makita circular saw to suit the style of battery and charger I already had. Again, I was not only delighted, I was amazed at the power and performance of the saw. Although I'm only cutting 4"x1" timber wall studs at the moment (length-ways and across), the convenience and time it's saving me is priceless. The supplied blade with the 'saw-only' purchase has remained sharp and complements Makita quality.
I've reserved my original Makita-branded battery (plus a spare pattern battery) for the drill, and bought two extra pattern batteries specifically for the saw. If you're a slow-but-steady worker like myself, two batteries will probably suffice (one working, one charging - takes 20 minutes). I haven't compared the original Makita battery with the pattern battery when used with this saw, but I read elsewhere a comment from a professional user stating no noticeable difference whatsoever - and they're almost half the price of the originals. The saw will also work the batteries harder so perhaps wiser to go for pattern batteries for the saw.
Finally, the supplied blade is 24-tooth for general work, but alternative types and brands are available. Be sure to go for the thinner blades better suited to battery-powered tools (typically 1.5mm or 'thin'). For compatible blade specifications, the manual states the blade diameter (165mm) but doesn't state the shaft/hole diameter - which is 20mm to fit this saw.
By all-means Google the reviews, but for me my search for a battery-powered circular saw is over. I'm absolutely certain you will not find better than this - or the Makita brand for that matter.
UPDATE:
Almost a few months on and I've used the saw and it's adjustable features extensively, additionally sawing full 2.4m lengths of 18mm hardwood ply (90-deg and 45-deg cuts) to the studs I mentioned above. The supplied blade still appears to be as sharp as the day I got it so should last quite a while if not abused.
The pattern batteries also continue to perform very well, though I did notice that they do not feature gold-flashed contacts like the original Makita battery. Also, with one of them, it initially seemed at bit 'tight' at-first and I had to reinsert it into the charger a couple of times to avoid the charger indicating a battery fault. Possibly a poor contact or tolerancing issue and no problems since, but worth a mention should you choose to use the non-Makita alternatives.