+1 to the 2.5 gallon BIAB batches. I started out AG (2 months ago, lol) with a 2 gallon pot, a 4 gallon pot, and a 5.5 gallon pot, and did about 5 3 gallon AG batches. I just upgraded to a 7.5 gallon ported kettle, but I still do a fair number of 2.5 gallon batches. (I've done a total of 18 brews since the beginning of the year - I think I have a problem. )
Half batches are awesome because you get about a case worth, and you can do lots of different things provided you have the fermenter space - I have 2 6.5-gallon buckets, 3 3-gallon glass carboys, and (now) 2 5-gallon white Lowes buckets. AG takes a fair bit more finesse to hit your mash temperatures and gravities, and practicing with smaller batches is absolutely worth it.
I've actually only done one 5-gallon AG batch, and that one I decided to split it and do two half batches with different yeasts - I made a Schwarzbier and a Black Saison (I call them Arnold and Jean-Claude ). I've done 3 5-gallon partial mashes, where I got half or more of my fermentables from grain, supplementing with extract as needed.
I'll probably switch to a more "standard" cooler-mash-tun, partly because I feel like that's a better long-term option than sticking with paint strainers forever, but the paint strainer bags are definitely one of the best and cheapest pieces of equipment I have. I have 2 48 quart rectangular coolers, so I really have no good reason *not* to build a real mash tun. In terms of speed and effiency, though, I really have no complaints at all.