I've been to Costa Rica three times now – the last being for three months in June 2005.
The good stuff: I've been to almost 30 countries so far and this place has some of the nicest people and best geographical diversity. The jungles here are immense with a great set of active volcanoes and waterfalls, and littered with monkeys, iguanas, parrots and blue butterflies. One of the things I like best is driving over the mountains and seeing the jungle from 6,000 feet up. The beaches range from super fine black volcanic to blinding white on the Caribbean side. Some good food in the big towns and shopping is nominal. There's crime in the big city and some petty stuff in the other towns but very safe overall. Immigration is cool and you just have to leave the country for 72 hours every 90 days.
The NOT so good stuff: Hotels, food, car rentals, and real estate are getting very pricey. The rainy season is wet but usually for only an hour a day. The lack of road signs and slow buses on windy mountain roads when it's pouring rain can be frustrating. Also, I was infected with the Dengue Virus while in the Pacific side and it was one of the most excruciating things I've been through (see List of Bad Travels blog).
Where to go: Wow, this could be a long list so I'll try to keep it short. Tamarindo – very touristy beach town with good food, casinos, and great surfing. Manuel Antonio/Quepos/Osa Peninsula – is a must see for first timers. Huge national park with lot's of animals and great beaches. Jaco is another beach town that is not quite as touristy but working on it. This town has great food, cheap hotels but is full of prostitutes and drug dealers not to mention the police are usually stopping tourist to see if they have their passport. Still a safe town – don't get the wrong impression. Monteverde/Santa Elena – gorgeous area, don't miss it and it's close to Arenal which has a great lake and an active volcano. Puerto Viejo is one of my favorites on the Caribbean side and has great food, extremely cheap hotels, and is the most laid back town in the country. The nightlife is great with Reggae bars and outdoor theatres. You can take a great two day trip across to Bocas del Toro on the Panama side. This place is awesome and worth the cab and ferry rides. I missed at least 12 other towns but unless you're going to be there for a month this is more than enough.
Where NOT to go: Of course the capital San Jose – however it is the only place for shopping and shitty American fast food. Puntarenas and Puerto Limon are the dirty port towns on the Pacific and Caribbean side, respectively.
After Costa Rica I took a bus to into Panama (see blog).