Greece is beautiful. The mountains are beautiful, and the sea is the most amazing color of blue that I've ever seen. Despite the amount of garbage that the Greeks dump in their waters, visibility is still amazing.
Not surprisingly, Greece is full of Greek and Roman ruins. Most of them are VERY ruined. A few are in good shape (such as the Acropolis), albeit covered in scaffolding. Museums are terribly documented, and they rarely have anything labeled in English.
Contrary to popular belief, Greece is NOT always warm. I arrived in mid-March, and it was freezing cold, both the air and water. In Athens and the Peloponnese, air temperatures were around 9-11°C (~48-52°F). I put my toe in the sea a few times, and it's amazing that I still have my toe, given how cold it was. When I cycled into the mountains (which are only about 1500m high), I saw snow by the side of the road. I got hailed on twice, and rained on quite often. At the end of my stay (late May), however, Greece was HOT and dry. You could comfortably swim in the sea, and you might well be hot even in shorts and a t-shirt.
Greece is NOT as cheap as I expected. It's cheaper than northern Europe, but it's not the bargain travel destination that I hear it used to be. Youth hostels cost $6-10/night and have virtually no facilities (no kitchens, for example). A double room in a hotel/pension will usually run around $20/night ($10/person for two), and significantly more in high season in the touristy areas. You can sometimes bargain them down, but don't count on it. Camping can be cheap if you have several people, but if you're alone, it's cheaper to stay in a youth hostel, as they charge BOTH by the person AND by the tent.
Personally, I love Greek food. Some people find it too oily (they use a lot of olive oil), but it's very healthy. It's possible to get along as a vegetarian in Greece, but you'd have a difficult time as a vegan (no dairy products). Food is "medium-priced," with an average meal in a restaurant running about $10-12. Eating on the street (Souvlaki vendors and the like) will cost more like $3/meal. Bread is very cheap and good (~$1 for a kilo), though sometimes not entirely fresh.
This is the reason I will never go back to Greece. I hated the people with a passion. I went expecting them to be friendly and hospitable, and instead found them to be rude, selfish, arrogant and totally inhospitable. I was in Greece for two months, and I met less than 10 people whom I liked at all. My impression of the Greeks is that they care about nothing but money, and will not do anything for anyone unless there's something in it for them. I tried very hard to learn Greek, and was never complimented on my Greek (unlike in Turkey and Italy). In fact, I stopped speaking Greek after I discovered that, if I spoke English only, they would speak English, but if I admitted that I spoke any Greek, they would stop speaking any English at all and forced me to try to speak more Greek than I was capable of. No matter how beautiful this country is or how fascinating are the ruins, I will never return to this place which I call "Land of the Rude People."
I have, however, met a number of people who have really enjoyed Greece. Of those, however, 95% have admitted that the only places they visited were the touristy islands (Mykonos, Ios, Santorini, Corfu, etc.). So, in fairness, I must say that if all you want is a beach holiday, it's certainly possible to have a good time in Greece. Lots of Sun, Sand and Sex, mixed with a lot of alcohol, and some people call that an ideal holiday. The people on those islands can be very friendly, too; they hardly have a choice, given how much money is coming in from the tourists.
Greece is not a third world nation, but they're not very clean, either. If you're at all prissy about dirt or other people's "germs," you should at the very least bring your own sleeping bag or sleeping sheet and a pair of shower thongs.
Greeks have not yet become aware of the need to deal with their trash -- a distinct problem caused by "modernization." When you round that beautiful mountain bend, you'll discover that the Greeks dump all their trash wherever they feel like it. They don't have landfills, so they just take it all in a truck up into the mountains and dump it on the most convenient hill, sometimes going so far as to set it on fire. They throw their trash on the ground as they walk, or out the window as they drive.
Greece is VERY mountainous. When you remember that the ancient peoples of this region fought wars over these mountains by FOOT, it's a wonder that anybody ever survived, let alone prospered. From a tourist standpoint, though, the terrain is beautiful. In many places, snow-capped mountains, covered in olive groves, seem almost to plummet straight into the sea. Lemon and orange groves combine with wildflowers to lend beautiful, vibrant color the landscape.
Greeks drive like mad, particularly in the cities (Italians are infamous for maniacal driving, but they don't hold a candle to the Greeks or the Turks). Fortunately, they also give incredibly wide berth to cyclists. Sometimes I was worried that they gave me TOO much space, going completely into the opposing lane of traffic, at high speed, around a blind mountain corner. Road conditions aren't as bad as you might think, given Greece's lack of prosperity, but the European Union is funding most of it, so it's not as much of a surprise. You will, however, probably run into some spots where the roads leave quite a bit to be desired. A mountain bike is not at all necessary, but may be advantageous if you want to cross some of the lesser-traveled mountain paths, which occasionally turn into dirt farming roads.