Overall, they are excellent pistols. They have come a long way since their 1st Generation Sigmas (the "Enhanced" versions are the 3rd Generation).
How old are your parents? Do they have any problems with their hands, arthritis or anything like that? The only problem I could see with that would be the heavy trigger pull. The pull is a DAO at about 12 pounds.
What you may want to think about is a S&W 5906 or 6906. You can buy used ones fairly inexpensively, and the trigger pull may be better for someone with weaker or arthritic hands.
Here's a link to Mark at Summit Gun Brokers, who has used 6906's for $289 (probably the same price you can buy a new Sigma for). He's a great guy to deal with, and the 6906 would probably be better suited for your parents. If you are worred about the magazine capacity (6906 mags hold 12 rounds), you can buy a 59xx Series 15 round mag and use a grip adapter, available from either Brownell's or directly from S&W.
http://www.summitgunbroker.com/736225.html
Other than the trigger pull issue, the new Sigmas are excellent pistols. I've owned or been issued eight of them, and in tens of thousands of rounds between them, I've had only one malfunction. One FTE in a 1st Generation SW40F that I had over 8000 rounds through. I've never had a malfunction in an Enhanced Sigma.