It is actually a good time to buy investment properties in many areas as rents are going up and prices are low.
It will depend on your reasons for selling.
Passive income is the best kind of income. There are many people that can only dream right now of being in your position. If your situation allows you to hang onto the property, I recommend doing so. But like many have said already, it really depends on your situation and your desire to continue the work required of landlord.
You need to sit down with your accountant or your calculator and assess what you have into the property, what you can sell it for and assess the income vs. the costs to keep it, including property taxes, maintenance and income taxes. Then you need to assess the intangibles of being a landlord. Fiscally, right now the prices to sell are going to be lower than in the future and rents are extremely high right now. So as long as you are making money and have happy tenants who pay, you may want to consider keeping it.
It's a question that only you can answer. As my colleagues have said, a lot depends on your situation.
1. What are your reasons for selling right now?
2. If you financed the property, will a sale bring you enough to cover the outstanding balance?
I think those are your first two big questions to answer. If selling is the result of those answers, then talk with your tax accountant and make sure that side makes sense.
Do a pros and cons list. Then, if you're looking to sell, consult a Realtor to get market information and a great marketing plan.