I found the Sigma Shark 635 watt PSU to be a pretty nice power supply. With it's aluminum case, exterior fins, twin intake fans, and venting design, it has a pretty unique look among power supplies. The LED fans emit a nice blue glow and were not very loud during my testing.
Though not modular, the cables are nicely sleeved and finished and are plenty long enough for a large case. There are plenty of connectors for most applications.
I did have a concern with the Shark's +12v though. 11.91vdc is within the allowed variance, but a little close to the limit. My well-overclocked rig never took it below that voltage and it caused no problems whatsoever, but I think I would be a little nervous with a pair of 8800s or the like. It is possible that this was an isolated incident, but generally, PSUs of the same design perform alike.
I would probably have some problem recommending this PSU for most SLI applications. But, to be honest, I have problems recommending most PSUs under 750-800 watts for SLI use, especially on an overclocked rig. I feel that 500-600 watts just doesn't give you the headroom you need for nice, clean +12v in stressful situations.
Also, I have to note that the PSU got warm enough with my rig at near idle to speed the fans up. It's no really big deal, just something I'm not accustomed to. I suspect in an SLI application, the fans would run faster much of the time.
But, I would have no problem recommending the Sigma SP-635 for most any non-SLI application, and possibly lower end SLI applications. For $99.99 USD at a popular online retailer, the price is at the lower end of 600+ watt power supplies, which puts it in the category of economy PSUs.
I would have concerns using in most high-end SLI applications due to low +12v voltage
The Sigma SP-635 earns the OCIA.net Seal of Approval.