I have had a lot of experience with burlap.
Re: Burlap in general. I would not use an existing burlap bag for anything except yard supplies.
If you want to make something with burlap, buy it at a fabric store. Do not buy the kind on a bolt, because the fold line will never iron out. Get the kind that has the full width of burlap on a roll, like upholstery fabric.

Re: Washing. NOT A GOOD IDEA.
If you do choose to wash the burlap, you MUST sew the edges beforehand. Use a medium stitch length (10 inches per inch/10 per 1.5 cm). If you drape rhe burlap over a clothes line and just rinse with a hose, you might be able to clean it somewhat. Burlap does shrink. DO NOT use a washing machine. The fibers shed a huge amount and will clog up your machine and your plumbing. You will also never be able to iron the wrinkles out completely, because the natural oil in the jute has been washed away.

Re. Cutting. You must clip the selvedge/selvage (originally from the words "self edge") at the desired measurement. Then choose one "thread" and gently pull on it, across the width of the weave. The fabric will gather. Work this thread out of the weave. It will take patience, and it will break a couple of times, at least. This is the only way to know if you are cutting burlap on a straight line. Cut on the line revealed by removing a "thread."

Re: Messiness. Extreme. Vacuum thoroughly. Wipe down hard surfaces with a damp paper towel, or other disposable item. Wash your clothes before wearing them again - burlap fibers are extremely itchy.

Re: Other disadvantages. Dark colored burlap fades, and light colored burlap oxidizes and darkens over a period of six to eight months.

How do I know this? I learned by covering my classroom bulletin boards (tack holes will not show). Burlap also helps keep sound from bouncing off the walls, so the room is quieter.

Re: Burlap curtains (The following information is from www.wisegeek.com.) Burlap curtains make an interesting and unusual window treatment. Their rustic and rough-spun look will complement a variety of informal settings, from recreation rooms to college dorms. Burlap looks best among other natural fibers and textures, but can also provide an interesting contrast when coordinated with smooth, modern designs and surfaces. Burlap's pluses include its relatively low cost and ability to provide filtered light. Potential cons in choosing burlap curtains include a limited range of color choices, fabric width, and potential shrinkage, as well as dustiness, and smell.

In settings such as a rustic farmhouse or mountain cabin, burlap's highly textured surface coordinates well with country-style furniture and natural materials. An exotic touch can be added to a family or recreation room by using burlap coffee sacks bearing their original stencil marks. Coarsely woven burlap allows some light to pass through it, making burlap curtains a good choice when they do not need to block all exterior light while providing privacy.

Since burlap is usually made with natural hemp or jute fibers produced with little or no pesticides, it is an eco-friendly window-covering option. Extremely durable burlap will last indefinitely and is resistant to tears. Burlap is also extremely economical, making it a good choice when decorating on a strict budget.

Due to their rough texture, burlap curtains tend to hold dust and can be somewhat difficult to clean. They can also shed a considerable amount of fiber. Burlap shrinks significantly during laundering and residual shrinkage should be factored in when sizing curtains. Burlap also lacks the soft textures and attractive drape of other textiles used in making curtains. Due to its lack of drape, burlap will be more attractive when used to cover smaller windows rather than in floor-to-ceiling treatments.

Burlap can be hard to find in attractive decorator colors and may require some special sizing for windows. In some settings, however, its natural, brownish color will coordinate attractively with other shades. As a natural fiber, burlap can be readily dyed in a wide range of shades. Depending on the type and color of dye used, the curtains may fade during sustained exposure to direct sunlight. Burlap is typically sold in widths that are narrower than traditional drapery fabrics. As a result, the process of assembling draperies can involve more sewing and seams.

While burlap is known for its ruggedness, burlap curtains can become brittle and stretched out over time. Some burlap specifically produced for decorating purposes is treated to minimize brittleness, fiber shedding, and sagging. Due to its earthy odor, burlap fabrics are also sometimes given a deodorization treatment.