Not all Linksys routers can be reflashed with DD-WRT. Additionally, DD-WRT is not limited to Linksys - it works on many other routers and AP's, many of which are arguably superior to the Linksys hardware.

Many folks - myself included - are of the opinion DD-WRT has gotten too complex, and has it's own set of issues. Given the choice between the stock firmware and DD-WRT, I'd still go with DD-WRT - but there's another option.

Tomato firmware has a pretty big following of it's own - and I've found it to be far more stable, better performing, and more useful for my needs than DD-WRT. In particular, it's a superior choice if one is using VOIP - it has much better QOS than DD-WRT, allowing your voice calls to work near perfect regardless of what else is being downloaded/uploaded.