Hi Wolf,
My wife & I were in Guyana about a year ago, primarily looking for birds. In Georgetown, a really good guy to hook up with is Andy Narine with Guyana Feather Friends (
http://www.birdingguyana.gy/). I think he's pretty much known as being as knowledgeable about Guyanese birds as it gets - granted, that's a bit of a small pond, so to speak. We also spent time in the Rupununi, at Rewa village -
http://www.rewaguyana.com. Rovin Alvin is an excellent guide there, but if you try to contact them, you must be patient in waiting for a response. We liked this place the most. We also stayed at Rockview Lodge for two nights, which was a lot higher end than Rewa, but had some weird vibes. Sadly, we only had 10 days in country; we'd have liked to have stayed longer. We saw upwards of 300 species in that time, with over 200 in 3 days in Rewa, including a Harpy Eagle nest.
Guyana isn't really that easy to get to, or around in, which I think factors in to why it's not that popular; it's only got 700,000 people total, and 90% of them are on the coast. From a birding standpoint, if you're looking for endemics, there really aren't any, although Andy thinks that there's a Piculet sub-species endemic. From a safety standpoint, we always felt secure, but we also didn't venture into areas that looked dodgy, and didn't walk around Georgetown after dark. It's a common-sense thing, really.
I agree with Bill that Guyana isn't cheap. A lot of this has to do with the fact that it's hard to transport goods to the interior. That may be changing soon, however: about the time we were there (Sept. 2009), a bridge linking Bon Fim in Brazil and Lethem in Guyana was completed, and the Brazilians are hoping to complete a road from there to Georgetown, to replace the current unsealed road, which would give them fast access to the sea. This will no doubt bring many settlers to the interior ... probably good for Guyana's economy, but definitely not good for nature or the laid-back lifestyle of the interior.
Best of luck, Jim