It's not only OK to enjoy God, it is commanded! It's the POINT. It's our purpose. It's what God desires!
"Serve the LORD with gladness!" (Psalm 100:2, ESV) We are here commanded not only to serve God, but we are told HOW to serve God (with gladness). What an awesome God!
There is sufficient reason to have joy in God, in light of what he has done for us in Christ:
"Bless the LORD, O my soul, and all that is within me, bless his holy name! ...as far as the east is from the west, so far does he remove our transgressions from us." (Psalm 103:1, 12, ESV).
But more than what he has done for us, WHO HE IS is enough reason to rejoice in God (not in his gifts):
"Delight yourself in the LORD..." (Psalm 37:4, ESV)
"One thing have I asked of the LORD, that will I seek after: that I may dwell in the house of the LORD all the days of my life, to gaze upon the beauty of the LORD and to inquire in his temple" (Psalm 27:4, ESV).
The beauty, glory, perfections of the Lord are enough!
"Earnestly I seek you...my flesh faints for you as in a dry and weary land where there is no water.
...there is nothing on earth that I desire besides you." (Psalm 63:1; 73:25, ESV)
Don't forget his benefits (Psalm 103:3)! Consider the goodness of the Lord (Psalm 107:43)! Think about how amazing our God is, and ENJOY him! He has not only given us what we need, but he has given us things to enjoy. These things to enjoy are made into idols by some (worshiping the gifts rather than the Giver); but a proper enjoyment of these things holds them loosely and results in thankfulness and praise to God. It is ok to be happy, and to enjoy the things God has given us - all the while rejoicing in GOD above his gifts and desiring him and valuing him as the Treasure of our hearts.
"You cause the grass to grow for the livestock and plants for man to cultivate, that he may bring forth food from the earth and wine to gladden the heart of man, oil to make his face shine and bread to strengthen man's heart." (Psalm 104:14-15, ESV)
In the previous verses we see God giving us bread and food (what we need), but going beyond that and giving us things to enjoy (wine to gladden the heart and oil to make the face shine).
Even our future hope of glory in God is pictured by Jesus Christ as JOY: "Enter into the joy of your Master" (Matthew 25:23, ESV).
Hallelujah! Glorify God BY enjoying him forever (John Piper, Desiring God). No matter what your circumstances, serve the Lord with gladness. Call to mind that God has taken away hindrances to our enjoying him, by crushing our sin in Christ on the cross and rasing him from the dead. Those who reject Christ will never know the joy of God. But all those who are in Christ, serving him faithfully, will enter into the joy of their Master..forever!
Let us enjoy the gifts of God with thankfulness and praise. And let us enjoy, above the gifts, the great and awesome God who is our Treasure! Let us cast down idols and delight ourselves in the Lord!