It's a great game. It's been a long while since I last played SH3, but I'd say it's like SH3 with better graphics and some new features (and some lost). There are mods to make you feel at home though, like the TDW's UI which has a sh3 style. I've even imported music from SH3, and it at almost feels like the same game now.
On the flipside it's more demanding on the hardware and I sometimes get random ctds (as often as every 4-5 hours) so saving the game at least once an hour is very important.