Dear Gus,
I can vouch for Lukas' books and guiding services. I spent 2 days in Bialoweiza and 3 days at Biebrza in mid-May last year. We used both books which are very detailed and entertaining, and and available as pdf so you can carry them on your smart-phone. Of course they are recent as well (the Bialoweiza one was completed last year and the Biebrza one 2008) which means the gen in mostly spot on. In terms of layout and quality of information they are by some margin the best "where to watch birds in..." books I have used.
We also booked a guide (Arek Szmyura) through Lukas to help find woodpeckers and other sp. in the forest. This was invaluable -- there's no way we would have found Pygmy Owl, Three-toed or White-backed Woodpecker, or Red-breasted Flycatcher without him in the time we had available, and he also helped us find Hazel Hen and Grey-headed Woodpecker. Furthermore you are not even allowed into the Strict Reserve without a licensed guide. Though the only bird we had there and not elsewhere was Three-toed Woodpecker, I would still recommend making the effort, partly because it is an amazing experience to be in such a beautiful, undisturbed forest, but also to support local eco-tourism.
We saw Aquatic Warbler on 15th May at the well-known site Dluga Luka, so your timing should be ok (note also that Lukas' guides give the median arrival date of migrants!). Much to our surprise we had 2-3 birds there singing at noon! We had Hazel Hen at site 6 in Lukas' Bialoweiza guide, about 200-300m from the small parking area at site 6.1, though much of this trail is apparently quite good for it. We had to be very patient, and I appreciate we also had quite a bit of luck to see one in the middle of the day. I think early in the morning you have chances to disturb them feeding on the trails. We struggled with Black Woodpecker (Lukas gave us gen for two or three sites all of which we dipped at). We would have tried the Red Bog (site 17 in the Biebrza guide) but earlier that day we had a chance encounter in the forest on our way back from Kapice -- various species including Hawfinch and Black Woodpecker were feeding on the roadsides.
We didn't see (or look for) Bee-eater or Roller. they apparently arrive mid-May. Lukas would have better info than me.
It's worth mentioning that though we rarely felt "crowded", most places we visited there were other birders. This has the advantage that gen gets passed around while in the field, or over meals and/or beers in the evening. At that time of year there are various tour groups, and we in fact met up with Lukas himself at the Wejmukta Inn, and also with Marek Borokowski who was doing a week-long tour of the marshes and forest.
A full report of my trip is at
http://www.robots.ox.ac.uk/~ian/Birding.
Cheers, Ian