So I went ahead and purchased the Hayward T-Cell-940 (40K gal) at my local Pinch-A-Penny last weekend. First off, it's working too well, I overdid it and my first reading went from 0 to 8 ppm after a few days, so I turned down the %. It was the $599 list price with a $50 rebate. It was buy then or purchase more liquid chlorine and research another week.
My reasoning is based on the generics having lesser warranties but also a lesser chance of honoring their warranties, if they're even still around. My last cell was the compupool (now $300 with 3 yr warranty), which had to be replaced less than a year in, and with tedious records and paperwork sent, and over a month to get it. The reviews on it are now really bad, and some pool supply places have stopped using them. I also saw the Hayward online for $549, but with the 3 yr warranty. Plus the model i received is called "Extended Life (supposedly 25%), so it basically is $137.25 per year if it lasts the 4 years.
My motherboard is pre 2009 so I'm limited with certain cells too (I replaced the current limiter myself years ago to save the board).
Most generics came to about $100 per year of warranty. So for an extra $37 per year (or probably less if it lasts longer than 4 years), I have peace of mind, and the clincher was that my pool store not only can test the cell (they couldn't test my generic), but also honors the warranty. Anything wrong, I bring it to the store and they handle it. I regularly bring my water to be tested there anyway, so I felt this was the best deal for me.