I’ve done it 4 times now, and it takes 5 minutes to drop in DD-WRT on a WRT54GL. The biggest benefit? Raising the Transmit power of the wireless signal 3-4x’s (safely) the default. Allowed work’s entire building to be covered by a strong signal, when previously they could not reach all corners with ANY signal.
Past that, the other benefits really depend on your needs (VLANs, scheduled access, bandwidth restrictions, etc.) Personally, I have no need for all that at the moment.