I have had huge successes with haste towers in early-mid game. When it comes to actually placing them, I've wound up putting them in areas where they can only attack creeps for brief intervals before they go out of range. That way, their damage is done to each creep rather than just tunneling a single creep, and they build up speed before entering the larger portion of the tower's range (see diagram below). Coupled with the new geyser towers it works great. Also, since haste towers always attack the closest unit, placing them in such a manner that they prioritize the unit furthest along in your base works well. The red circles represent the haste towers and their "general" ranges. I'm sure there are better locations, but these worked out well for me. The map I used is the map for flash element TD, so it's a bit different, but I hope the idea still gets across. Ignore the white spots.
I built two geysers near the spawn and upgraded the second one to a haste tower if that helps you with regards to timing. I always play with at least three random players so I'm forced to build close to the spawn, but maybe haste towers around position 2 would work well.