There are several nice things to be said about reserves. It gives you the chance to safely hold your forces off the gameboard when you face a heavy alpha strike army. I also allows for outflanking and deepstriking forces to hit the gameboard often providing chances to really do some damage. There are decent strategies about holding troop choices off the board for late objective gathering, and probably several more that are not coming to the top of my thinking at the moment.
What I dont like about reserves, is that they can literally ruin your game on bad dice. While bad dice can ruin your game elsewhere, your reserve roll is a single dice roll per round for a unit. One bad die roll and done. Example. Last battle report I did, the greater daemon came in at the bottom of round 5. Literally if the game had ended after 5, it would of been a complete waste of points. I see this happen alot with reserves. Players playing with large point cost units that dont get to play until the bottom of 5 and the game ends. Pointless.
I prefer not to play the reserve game unless I have some way of manipulating or controlling it. Getting a +1 to reserve rolls or outflanking re-rolls, or even the latest Webway portals (allowing to place a board edge somewhere).
Fifth Edition with random game lengths really can screw your game over very quickly with reserves. I find it great fun when someone without any control over reserves decides it's best to reserve their army against my IG lists because he is afraid of that first round of shooting. It generally means I get to piecemeal his army. My full points against whatever he can muster out of reserves.
Reserves are not reliable. If you design your list to take advantage of reserves, then by all means it's part of your strategy. Not having a means to control them somehow though, will cost you in the end, and some games you will simply lose because of it.
The Point: Don't do it. Dont reserve your troops unless your strategy from list creation calls for it. Even then, take those ideas with a serious grain of salt unless you have ways to manipulate the ways your reserves come into the game.