Well, it is a switch - that's what it has been designed for 
OpenWrt works pretty well on rtl838x, the basic features (VLAN management, etc.) are working pretty well and DSA allows access to quite a few advanced features (via iproute2, ip, tc, etc.), but luci hasn't really been optimized for switch use (beyond VLAN management), yet. Running OpenWrt you do gain real security support (I don't think we'd need to discuss the security of OEM's kernel v2.6.19 and it doesn't stop there) and a uniform configuration interface between your devices (routers and switches), but some of the advanced features of a 'typical' managed switch might not be easily exposed to the webinterface so far. (Don't get me wrong, the basics of VLAN management are working nicely with luci, you can easily define different VLAN groups and configure your ports which to belong to, which to tag/ untag).
But, it works, really well, right now - certainly good enough to give it a try (and even if you might not plan to flash OpenWrt for now, retaining the option for the future (by choosing supported hardware) is certainly a good idea).
EDIT: the OEM management interface of switches from different vendors usually varies widely, which can often be quite confusing and illogical, with OpenWrt you do gain the same interface (and underlying DSA based configuration syntax) for all of your devices, that can be pretty valuable and save a lot of time/ head-scratching. Some OEM firmwares also tend to be rather neglected by their manufacturers, depending on obsolete crypto algorithms (e.g. SSL3) or JAVA/ flash, which can make it a nightmare to work with these devices, luci is getting updated with the rest of OpenWrt to meet current requirements.