You are obviously getting benefits from what you have brewed, so I would say yes it is Ok to drink it warm.  I found that I had to lie down after drinking it when I first started and I think that was a part of the healing process.  I could feel the effects going through my entire body.  Once I'd taken it for a week or so I didn't have the same reaction.

It is a long process to brew the tea if you follow the original directions.  If you do your first boil in the evening and then let it sit overnight, you can bring it back up to just below the boiling point the next morning and the majority of the time involved is in just waiting.  From what I gather, the herbs in the water for that length of time extracts the maximum benefit out of them.  I've found that there's a lot of water loss in the brewing process and a gallon of water makes significantly less than a gallon of tea.  For this reason I'm very careful not to over boil the tea and have it at a simmer/boil for just the ten minutes and then take it off the burner.  This minimizes water loss.

I bought my first batch of Essiac from Mountain Rose and they are more reasonable than a lot of companies.  Since then though I've bought the individual herbs and mixed it myself.  I find that much cheaper.  While I still buy a lot of herbs from Mountain Rose, I find the Monterey Bay Spice Company to be cheaper and have even found one other cheaper than they are.  Price is important to me, but MBSC doesn't sell the pre-mixed, you have to mix it yourself.

Here's the recipe that I use: http://theherbs.info/recipe1.html