I'm from Ankara; one of the closest big cities to Kartalkaya. It takes 2 hours car driving with the new Dörtdivan route from Ankara to Kartalkaya.
But if you are located in Istanbul, it is around 3 - 4 hours with car driving.
For last 10 years, during winters, I'm spending most of my weekends in Kartalkaya, so I guess I know much about the mountain. I wish to share my observations and experiences.

Some key points about Kartalkaya:
- Both from Istanbul and Ankara, every day (weekends and weekdays) there are daily buses direct from to cities to Kartalkaya. They depart very early in the morning, they arrive at Kartalkaya around 9:00 AM, so you will not miss the day, you will catch it from the beginning. To return back to cities, they leave Kartalkaya around 17:00 (when the slopes close). To reach those daily shuttle bus companies, you can have their information from the hotel receptions (via phone).
- This gives you an advantage to come for a daily trip, without accommodating. Ankara people are more lucky because it's much closer and daily trip becomes less tiring.
- Kartalkaya is the best option to ski or snowboard in Turkey, when you compared with other resorts. Only competitor would be Palandoken Mountain in Erzurum (and also Sarikamis could be a different option too) but they are located at the most east part of Turkey, so going with plane becomes another cost.
- Kartalkaya is an expensive resort. Unfortunately. Every service in mountain is overcharged! But when you have a hunger to ski or snowboard or when you want to do something different for a weekend, it becomes a nice option, because it's closeness to big cities like Istanbul and Ankara. So you may don't care about the expensiveness.
- When the weather is clear, sunny, without fog, and the snow condition is good, you can have excellent time for a couple of days. More than 3 days would be much, because you would get bored to ride on same slopes everyday.
- If you wish to stay long, weekdays would be cheaper, it would be fair.
- If you are going to organize your only winter holiday which you waited for whole summer, I suggest you to go to Alpes or Bulgaria. (Bulgaria stays extreme cheap when you compared for any season.)
- If you wish to go with your family, when the kids are at semester holiday, (so, I mean the semester period), stay away from the super-overcharged semester prices of Kartalkaya, even Alps would be cheaper option with flights included.
Conclusion:
* Daily trip is a good option from Ankara and Istanbul.
* Weekdays are fair.
* Weekends are ok to stay for 2 days.
* Long stays including weekends and semester holidays are meaninglessly overcharged, and you will get bored to ride in short, same slopes.

Kartalkaya is divided into two parts. Kartal Part is the left part and Doruk Part is the right part of the mountain. Unfortunately, you cannot ski on both parts with the same skipass. Here I wrote the pros and cons for each choices.

Common issues for both parts:
- You can finish every slope in one day. So your riding time pass in a loop on same slopes.
- Both part has a snowpark, organized and cared by volunteer snowboarders, there is no halfpipe, but the snowpark conditions are good when you compared with the overall attributes of Kartalkaya.
- The longest nonstop route is about 2,5 km, and there about total 6 or 7 different ski routes.
- Prices for both part are same or close.

Kartal Part Pros and Cons:
- Better off-piste than Doruk. Off-piste has more pines and more steep parts.
- At the afternoon, sun is leaving the area later than Doruk Part.
- The apres-ski and cafe option on piste is worse.
- Old mechanics (teleskis and chair lifts).

Doruk Part Pros and Cons
- Longer individual slopes than Kartal Part.
- The apres-ski and cafe option on piste is better.
- Has a one new chair lift which is fast and comfortable (on the longest slope), the rest of the mechanics are old.

About the hotels for whom want to accommodate:

Kartal Otel and Grand Kartal Otel:
- Gives you the free skipass for Kartal Part as you stay.
- Very polite and well trained staff, with a nice welcome.
- Incredibly delicious meals in both hotels (Kartal is even better than Grand Kartal about meals). They are using the all advantages of rich Bolu Mountain resources, so food they serve is very good.

Dorukkaya Otel:
- Gives you the free skipass for Doruk Part as you stay.
- Rude and uneducated staff, uncared service.
- Meals are catastrophic! Really bad when you compared with other hotels. (Still, foreigners could enjoy the meals, I guess they find it good when they compare with their own cuisine culture)
- The only positive thing I could say about this hotel is: the atmosphere of the lobby and apres-ski is more warm and enjoyable than other hotels.
- The only thing, the hotel owners' care is money they earned, they see the customers at the walking wallets. They don't care about the quality of their services.
- I guess, the only reason why people would chose Dorukkaya to stay is for to ski in Doruk Part.

Tips for daily visitors:
- Both Doruk Part and Kartal Part is competitive about slopes, they both have different advantages and disadvantages. Try and experience both of them in different days. But don't even think about purchasing both skipasses for the same day, it would be a meaningless expense, just choose one.

Have a nice trip ;)