I took the 434 bus last month when I was in SIntra. I didn't have any major problems - I was lucky that one was leaving just after I got off the train in Sintra so I just hopped on and bought the day ticket. The only problem I had with it was that I visited the Moorish Castle second after Pena Palace, then got on the 434 at Moorish castle (which is before Pena), and when the bus got to Pena, it stopped for about 20 minutes. A few of us on the bus didn't realize this was going to happen (headed back to Sintra) and we just waited. There was another 434 bus right in front us for 2-3 minutes - plenty of time for us to have gotten off and gotten on that bus - but the idiot driver never told us!!!!! So waited almost 20 minutes longer than we could have to get back to Sintra.
I recommend not buying your Pena Palace tickets at the Pena entrance. (You can buy them online!) It was a mob scene there - at both the ticket machine lines (which only offer tickets for park + palace; I wanted park-only entry, which lets you walk around the palace just not go inside) and at the huts. I probably would have done better going to the Moorish Castle first (probably much shorter lines - you can even go to the 2nd Moorish castle ticket window about 5 minutes ahead of the first one).
The Pena Palace is amazing (from what I saw on the outside, anyway - seen enough palaces and castles inside, don't need to see another one). The park you walk through to get up there is nice but not worth spending much time in. I'd hustle to the palace, see it, and then move on from Pena and not really bother with the park. Truly the highlight for me of Sintra was Quinta da Regaleira, but I was there only for a day so didn't get to see everything in town.
Note that when you catch the train back to Lisbon, some of the trains go back to Oriente station not to Rossio (where most tourists would want to go), so watch that. It is possible to take a train to Oriente and change to one to Rossio closer to town - that's what I wound up doing.